Reduce upper limit of stored bitmaps to 32

This helps reduce the max amount of memory needed for storing images.
While we do lose some support for incredibly long playlists by default,
we do ensure the current playing track from a playlist will always be
stored

Tag: #stability
Bug: 186706117
Test: atest BluetoothInstrumentationTests
Change-Id: I3003adc85f9d09654a2838444b215e9a1a3ed730