Only trigger a metadata sync if we don't have any cover artwork

This change makes our solution to the race condition of cover art vs.
the control channel/track changed updates less aggressive.

Without this, every folder change causes a redownload of the current
artwork. This is a terrible UX. We can instead let the Target side tell
us when the backing data has changed if we already have artwork.

Tag: #compatibility
Bug: 158030613
Test: atest BluetoothInstrumentationTests
Merged-In: I3eee51b2b5fe6d7d99cfe3e4b821327303be1e26
Change-Id: I3eee51b2b5fe6d7d99cfe3e4b821327303be1e26