Add synchronized tests for ContextualCallbacks

Post instrumented contextual callbacks and make sure that
they run on the correct thread.

There are three promise/future pairs that handle the
synchronization between the test and the main thread.

The test:
1. Schedules the task on the main thread and waits until it runs
2. Schedules another task and makes sure it isn't running (blocked
   behind the first)
2. Unblocks the first task
3. Waits until both tasks finish

Flag: TEST_ONLY
Test: atest bluetooth_test_gdx_unit
Bug: 330255035
Change-Id: I22adddf227e9fbd52fa6ec8783d105f635fe36b4