Add PlaybackState.ACTION_SET_PLAYBACK_SPEED

When "04fb2e74: MediaSession/Controller: Add playback speed related APIs"
was merged, the constant was not added.

This CL adds the new constant.

Bug: 174297519
Test: m update-api -j;
Change-Id: I698737bcfe641edd7b5ea07b4b385c81fe899994