Make transaction labels per device

Prior to this change, transaction labels (used for AVCTP commands) were
managed in a central location and shared between all devices. Only 16
labels were ever available for us before we "ran out" and stopped
sending requests. While this barely works for one device, this doesn't
work with multi-AVRCP situations.

This change moves transaction label management to the device struct such
that each device has its own set of transaction labels separate from
others. All transaction label functions were changed to be a function of
the device. Initialization of devices and labels was consolidated too.

Tag: #stability
Bug: 186240222
Test: atest BluetoothInstrumentationTests
Change-Id: If39d7f9ceb27469b9248927fbf5c92ab897b1e9e