Tighthen access qualifiers in BluetoothRouteProvider

- Tightened access qualifiers where possible: public -> package private,
  package private -> private
- Added static qualifier where possible to prevent implicit class
  reference leaking

Bug: b/255495104
Test: atest SystemMediaRouter2Test
Change-Id: I1bb1b67c76835168d4f1a738805da94e3a684d86