Per-test WindowManagerService instance

This CL starts using a WindowManagerService instance for each test method,
instead of sharing a single instance within a test class. It also starts
resetting mocks after each test method. With a MockTrack hack, it reduces
memory leaks happening in extended inline Mockito (b/123984854).

Bug: 123652272
Test: Pass all non-flaky presubmit tests in WmTests.
  $ tradefed.sh run commandAndExit WmTests \
      --include-annotation android.platform.test.annotations.Presubmit \
      --exclude-annotation androidx.test.filters.FlakyTest
Change-Id: Icbfd5c5d726e4c29a623682deac852c5b14667dd