Fetch Available Players on our own when they change

Prior to this change available players were fetched according to when
the MediaBrowser clients wanted the data. This is not sufficient for us
to keep up to data info on available players on the remote device. We
need to fetch them as soon as they're known to be invalid.

Tag: #stability
Bug: 187099538
Test: atest BluetoothInstrumentationTests
Change-Id: I9fa90826e4d9dc02fbc90be4e5a39b81a3562029