AVRCP: Only display "Not Provided" for the title of a song

The Artist, Album, and Genre fields will now be reported as an empty
string instead of showing "Not Provided".

Bug: 68812037
Test: Connect to a carkit and see that missing fields aren't showing.
Change-Id: I419b3d2f6a18dbecd3711cfd3dcb340be48ce7e4
(cherry picked from commit 2d3ee9a7)