AVRCP: Gracefully handle missing item attributes

The source of our metadata sometimes doesn't even give us basic title
information, which causes a NullPointerException when we try to fill the
Displayable Name.

Fill it with "<unknown>" when we don't know the data.

Add more useful debugging for GetFolderItems and GetItemAttributes.

Test: Tested with Audi and Porche car kit
Bug: 37657532
Bug: 37718715
Change-Id: I183f2b9c5714ebacabc8093de1c2bc166e323fae
(cherry picked from commit 89728a4d)
(cherry picked from commit 0f9c79e8)