Make URI based image support optional and off by default

URI based images require us to cache a local copy of an image we fetch
on our own. Applications that rely on this, especially for the now
playing list, can cause our memory usage to spike as we put those
images on our heap.

The easiest and most consistent way to handle this is to disable URI
based images altogether. We'll create an easy way to enable it for those
that want to experiment.

Tag: #stability
Bug: 186706117
Test: atest BluetoothInstrumentationTests

Change-Id: Icad1aecd283f613712c99cc357d3f5eef49cf570