Delete UIDGenerator cache files after unit tests

Add a UIDGenerator::unitTestTearDown() method that can find an delete
cache files created by UIDGenerator. Call that cleanup method from the
tearDown() method of the UIDGenerator unit tests.

Bug: 44850
Change-Id: If342586d93bbc2a926af2dc388325dcb7c345e37
Merge "Delete UIDGenerator cache files after unit tests"