Improve thumbnail generation utilities.

The existing APIs were pretty limited by only accepting a "kind"
value, so improve them to accept an arbitrary size, and offer a way
to cancel requests when no longer needed.

The older APIs were a mix of both public and @UnsupportedAppUsage,
so mark them all both public and deprecated so we can clearly steer
developers towards better options.  (The deprecated methods are
implemented using the new APIs internally for sanity.)

Use modern ImageDecode internally, which is more robust than
BitmapFactory.  Add CTS to confirm that we generate thumbnails of
reasonable sizes.

Bug: 119887587
Test: atest android.media.cts.ThumbnailUtilsTest
Change-Id: I4ca35569ad5c661b327a0cb24a48ebc21f6087b7