MediaRouter: enable transferring from/to BT devices.

The main objective of this CL is to enable selecting BT device.
For that it updates MRM and SystemMediaRoute2Provider such that
  - MRM.getRoutingControllers() returns a list including the system session
  - SystemMediaRoute2Provider marks routes as "transferable"
  - SystemMediaRoute2Provider sets provider id correctly
  - SystemMediaRoute2Provider handles transferToRoute()

Bug: 147979868
Bug: 147122575

Test: atest mediaroutertest
&& manually selecting phone / bt devices from output switcher

Change-Id: I2e2032fd6677f79b9f864c313c40846daa87f113