docs: Add note to getFrameAtTime() pointing to getScaledFrameAtTime()

Added a note to the various getFrame... methods saying that if you
don't need a full-resolution frame, getScaledFrameAtTime() is
preferable. Removed existing text suggesting you use those other
methods for fetching a thumbnail.

Staged revised doc to:

http://go/dac-stage/reference/android/media/MediaMetadataRetriever.html

Bug: 69804003
Test: make ds-docs
Change-Id: I54250b2d127987ef3c1633c011f5eac6b747372e