Don't use WallpaperPicker2 BuildCompat

packages/apps/WallpaperPicker2 has its own copy of BuildCompat,
supposedly to allow overriding the SDK version in tests, but that
feature is never used and having an extra copy causes more work
every time the platform version is updated or finalized.  Use
checks against android.os.Build.VERSION.SDK_INT instead.  Future
checks against development versions should use the offical
androidx.core.os.BuildCompat.

Bug: 186121492
Test: m ThemePicker
Change-Id: I6a30d63c10cea4c743d98bc778255b333796c953
Merged-In: I6a30d63c10cea4c743d98bc778255b333796c953