Common: remove GD related stuff from common

* Remove reactor, thread, handler, alarm, repeating alarm from common/
  since they are already copied into gd/os
* Extract thread and alarm benchmarks into gd:bluetooth_benchmark_gd
* Enable host side unit tests for system/bt in TEST_MAPPING

Bug: 129537494
Test: run benchmark, run unit tests
Change-Id: Id5d7592067bab3275976dfc710e4d751c43b9df8