Fix failing testOnPictureInPictureRequested

Fixes an issue where relying on a spy of ClientLifecyleManager caused
the test to fail with a TooManyActualInvocations error because
scheduleTransaction() was getting called more than once by mockito.
This change replaces the lifecyle manager spy with a mock.

Bug: 153709777
Test: atest WmTests:ActivityTaskManagerServiceTests
Change-Id: I5dfa780e28dabc4d634ef45667eb3688d07361b2