Handle multiple active bluetooth devices

When dealing with a pair of hearing aid devices are active,
a single "selected route" is not sufficient.

Instead, this CL introduces activeRoutes which can hold multiple active
bluetooth route and deal with them.

Bug: 152585170
Test: cts tested & manually w/ A2dp BT devices
Change-Id: I2c2b8967d9f221db73f9d3b9332de7c76d07a0f9