AVRCP: unify Get{Element,Item}Attributes response.

GetElementAttributes response and GetItemAttributes response share the
same format and require the same checks for length.

Test: play media on carkit, see media. especially with long items.
Bug: 32407250
Bug: 30571638
Change-Id: I8623e7d662f7a39112b7527b6f5ab63c5e32379c