Add Bitmap.CompressFormat#WEBP_LOSSY/LOSSLESS

Bug: 135133301
Test: Iadbd8cf3b69a150b9e38ad556392346e1bb27084

WEBP does not give clients explicit control of how to do their WEBP
encode. Add new formats that do. Update the docs for the existing
formats (and the new ones) to explain more precisely how quality is
interpreted.

Change-Id: I9583903c21ab2048fed8e7ed501ee8377ea5ba36