Check mBtRouteProvider for null before using it

BluetoothRouteProvider.getInstance returns null if
there is no BT adapter available (e.g. on Android Studio
Emulator) which causes a bad crash.

Bug: 152647633
Test: boot, play video in chrome
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: Ic52a164c348290463cbf1fba913a67d378b746d9