Fix null playstate update from players.

Some players will send a null object as parameter for
playstate change callback, without updating the
MediaSession. These events should still trigger an update
by using the Mediasession metadata if it is different.

Bug: 297191838
Tag: #feature
Test: atest BluetoothInstrumentationTests
Change-Id: Ic5a5b6a68e616af939a4e42d7d64476932dc38d3