hh: run all uhid exchanges in uhid thread

Introduce a new fd to be used for internal events between btif and
uhid thread. Use this new communication channel to serialize uhid
requests and response so all uhid related variables are only accessed
in the uhid thread.

Flag: com.android.bluetooth.flags.hid_report_queuing
Bug: 291522341
Bug: 345498287
Test: mmm packages/modules/Bluetooth
Test: see test in aosp/3003554

Change-Id: I38073d4ad064c9b30103a070efdad87353a8c9f5