[build] Check in Mono.Android.Apis.projitems, remove generator

[jonathanpeppers]

Description

Mono.Android.Apis.projitems was previously generated at prepare time by
GeneratedMonoAndroidProjitemsFile
(in xaprepare) from the
BuildAndroidPlatforms.AllPlatforms
list. Both had exactly one consumer — each other — so check in the projitems
as a source file and drop the generator plumbing.

Continues the xaprepare cleanup started in #11568 and #11580.

Changes

• Add src/Mono.Android/Mono.Android.Apis.projitems — content matches the
  generator's output byte-for-byte (validated by re-running the generator's
  exact algorithm and diffing); only the header comment now points at
  Documentation/workflow/HowToAddNewApiLevel.md instead of declaring it a
  "GENERATED FILE".
• Repoint the 4 importers (Mono.Android.targets <Import> + Inputs=,
  create-installers.targets, create-android-api.csproj,
  Xamarin.Android.Build.Tasks.targets Inputs=) at the new path and drop the
  Condition="Exists(...)" guards — the file is always present now.
• Delete GeneratedMonoAndroidProjitemsFile.cs and AndroidPlatform.cs
  (whole files — AndroidPlatform class and AndroidPlatformExtensions both
  become unused).
• Drop the new GeneratedMonoAndroidProjitemsFile() registration in
  Step_GenerateFiles.cs.
• Remove BuildAndroidPlatforms.AllPlatforms and the now-unused
  using System.Collections.Generic;. Kept the NDK constants
  (AndroidNdkVersion, AndroidNdkPkgRevision, NdkMinimumAPI,
  NdkMinimumAPILegacy32) — they still have consumers.
• Update Documentation/workflow/HowToAddNewApiLevel.md with the XML-based
  instructions for adding a new API level (replaces the old C# AndroidPlatform
  snippet).
• Narrow the build-tools/xaprepare/README.md blurb for
  BuildAndroidPlatforms.cs to NDK metadata.
• Fix stale doc comment in GenerateSupportedPlatforms.cs pointing at the old
  generated path.

Verification

• dotnet build build-tools/xaprepare/xaprepare/xaprepare.csproj — 0 warnings,
  0 errors after deletions.
• dotnet msbuild src/Mono.Android/Mono.Android.csproj -preprocess:pp.xml —
  preprocessed XML shows all 34 <AndroidApiInfo> items resolving from the new
  path. Same for build-tools/create-android-api/create-android-api.csproj.
• Byte-for-byte diff of a standalone re-run of the generator's exact code
  against the checked-in file: only the 2-line header comment block differs;
  body identical.

[Copilot]

Pull request overview

This PR continues the xaprepare cleanup by eliminating the prepare-time generator for Mono.Android.Apis.projitems, checking the file into source control instead, and updating all build/import points to reference the new location.

Changes:

• Check in src/Mono.Android/Mono.Android.Apis.projitems and update build/importers to use it directly (removing Exists(...) guards).
• Remove the generator plumbing and its now-unused data types from xaprepare.
• Update workflow documentation to describe the XML-based process for adding API levels.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.targets	Updates incremental Inputs to point at checked-in Mono.Android.Apis.projitems.
src/Mono.Android/Mono.Android.targets	Imports checked-in projitems and updates _GenerateAndroidApiInfo inputs accordingly.
src/Mono.Android/Mono.Android.Apis.projitems	New checked-in source of truth for @(AndroidApiInfo) items.
Documentation/workflow/HowToAddNewApiLevel.md	Switches “add API level” instructions to editing the projitems XML directly.
build-tools/xaprepare/xaprepare/Steps/Step_GenerateFiles.cs	Removes registration of the projitems generator step.
build-tools/xaprepare/xaprepare/ConfigAndData/BuildAndroidPlatforms.cs	Removes the API-level list, leaving NDK metadata.
build-tools/xaprepare/xaprepare/Application/GeneratedMonoAndroidProjitemsFile.cs	Deleted (generator no longer needed).
build-tools/xaprepare/xaprepare/Application/AndroidPlatform.cs	Deleted (type became unused after generator removal).
build-tools/xaprepare/README.md	Updates docs to reflect that API-level list moved to src/Mono.Android.
build-tools/Xamarin.Android.Tools.BootstrapTasks/.../GenerateSupportedPlatforms.cs	Updates doc comment to reference new projitems location.
build-tools/installers/create-installers.targets	Imports checked-in projitems file directly.
build-tools/create-android-api/create-android-api.csproj	Imports checked-in projitems file directly.

[jonathanpeppers]

/azp run

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