[fxl] make ThreadChecker BasicLockable

This CL makes ThreadChecker comply with BasicLockable so it can be
used in conjunction with static thread analysis. This allows e.g.
class members to be annotated with FXL_GUARDED_BY(thread_checker_)
to ensure that the thread is checked everywhere those members are
accessed.

TEST: added lock/unlock calls to existing unit test

Change-Id: I21264be75c8ed64ca71d40eb18d752021cad20b1
3 files changed