Fix flaky ActivityManagerServiceTest.

Ignore the runnable posted by the
UidObserverController as the test doesn't need it
and might result in unintended data access from
the handler thread.

Bug: 205035410
Test: atest ./services/tests/servicestests/src/com/android/server/am/ActivityManagerServiceTest.java
Change-Id: I65da69508f055f23692ff73bd436db399b2ee607