Slight adjustment of PLAY_ORDER behavior.

Play order is now automatically updated as playlist items are added
and removed, and callers can't make their own gaps.

Bug: 134683342, 151800650, 148995524, 151209792
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: Id749064c60dcbd2c02fd5b852dec3e67ba4969bb
1 file changed