MediaRouter: Set initial mIsBluetoothA2dpOn value

When BT is connected before MediaRouter is initialized,
the mIsBluetoothA2dpOn is not set as true.

Since the value is false, the 'currentSystemRoute' variable in
MediaRouter.selectRouteStatic() is always default route.
This leads to skip selectRoute(defaultRoute) which is called by apps.

The value is always false and not renewed, until audio routes changed.
This results apps cannot make sounds through phone speaker.

The CL fixes the issue by correctly assigning the value when MediaRouter
is initialized.

Bug: 189176702
Test: GMM can make sound via phone speaker.
Change-Id: I4a83f9441a8b3165b7dd668aeef12c25fe3817c0