Convenience method for obtaining thumbnails.

Obtaining a thumbnail for a Uri requires opening it with the slightly
obscure openTypedAssetFileDescriptor(), passing in "image/*" with the
right Bundle of EXTRA_SIZE to hint the target area on screen, and
defensively scaling any returned results.

This is pretty tedious to get right, so offer a convenience method
that does all this for the developer.  Internally uses ImageDecoder
to follow best-practices, and replaces older getDocumentThumbnail()
implementation by delegating to this one.

A future CL will teach ImageDecoder about how to read any embedded
EXIF thumbnails, including any required rotation.

Bug: 111268862
Test: atest FrameworksCoreTests:android.content.ContentResolverTest
Change-Id: I5c9c09ddf9b480f3b5c6ade0a078cccb68de2f2b