GraphicBufferSource: Add timestamp snapping toggle

The timestamp snapping logic for the slow motion recording can now be
toggled on or off at run time with the property
"debug.stagefright.snap_timestamps". If the property is set to any
value other than 1, GraphicBufferSource will not snap timestamps in
the slow motion mode.

Test: atest CtsMediaTestCases -- \
--module-arg CtsMediaTestCases:size:small

Test: adb shell setprop debug.stagefright.snap_timestamps 0
- Then do slow motion recording.

Bug: 135030072
Change-Id: I959f8640a932cb7a7b8bcdc97b15f7affe4d24ef