Add getBitmapCrop overload + add @TestApi

This is to improve test coverage, and add a new getBitmapCrop overload
for wallpaper picker only (see javadoc for its use case).

Flag: EXEMPT no logical changes (this adds a method that is not used)
Bug: 395679036
Test: atest WallpaperManagerTest
Test: presubmit

Change-Id: I166cc16b444f021db739cec896ae066a40268966