TryGetPlatformDirectoryFromApiLevel should fall back from android-{major} to android-{major}.0

[jonathanpeppers]

Problem

Starting with API 37, Google's SDK Manager installs the platform to platforms/android-37.0 (package name platform-37.0_r01), but TryGetPlatformDirectoryFromApiLevel only tries platforms/android-37 and fails.

This causes XA5207 for customer projects targeting net11.0-android37.

Current behavior

In AndroidSdkInfo.cs:

public string? TryGetPlatformDirectoryFromApiLevel (string idOrApiLevel, AndroidVersions versions)
{
    var id = versions.GetIdFromApiLevel (idOrApiLevel);
    string? dir = GetPlatformDirectoryFromId (id);  // \"platforms/android-37\"

    if (Directory.Exists (dir))
        return dir;

    // Existing fallback: minor -> major (e.g. android-36.1 -> android-36)
    // Guarded to NOT fall back for minor versions (dotnet/android#10720)
    if (Version.TryParse (id, out var version) && version.Minor != 0)
        return null;

    // Integer fallback: also tries GetPlatformDirectory(37) -> \"android-37\" (same thing)
    var level = versions.GetApiLevelFromId (id);
    dir = level.HasValue ? GetPlatformDirectory (level.Value) : null;
    if (dir != null && Directory.Exists (dir))
        return dir;

    return null;  // <-- XA5207!
}

There is no fallback from android-{major} to android-{major}.0.

Proposed fix

When android-{id} doesn't exist and id is a pure integer (major-only), also try android-{id}.0:

// After existing checks fail, try major.0 fallback
if (int.TryParse (id, out _)) {
    dir = GetPlatformDirectoryFromId (id + \".0\");
    if (Directory.Exists (dir))
        return dir;
}

Workaround

dotnet/android#11072 aligns the data so that platformID is \"37.0\" instead of \"37\", which makes the AndroidApiInfo.xml <Id> element 37.0. This means the first lookup will try android-37.0 directly.

However, the android-tools fallback is still valuable as a safety net for cases where the Id is just \"37\".

Context

• Fix API 37 platform directory name: android-37 to android-37.0 android#11072 (data fix in dotnet/android)
• Release build failed with "cannot find symbol android.app.KeyguardManager.DeviceLockedStateListener, android.service.chooser.ChooserSession.StateListener" errors android#10720 (the PR that added the minor version guard)