gtbs: Add Generic Telephone Bearer Service implementation

This adds Generic Telephone Bearer Service (GTBS) implementation
and exposes API that allows other TBS instances to be registered
by applications which want to expose their call control interfaces
over Bluetooth.

The GTBS aggregates call states of all the registered TBSes.
Each Call from GATT Client perspective is identified by a call
index assigned by TbsServerService, which is unique across TBS
instances and GTBS. From an application perspective, a call is
identified by the BluetoothTbsCall instance which has a unique UUID.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
test: Test: atest BluetoothInstrumentationTests
Change-Id: I284ddacfc8c7b48d633dd4097e3abf030bc1ef2a