Reduce flakiness of insets test in ActivityRecordTests

Because performSurfacePlacementAndWaitForWindowAnimator waits
for another thread, which can give the chance to run the callback
twice: executeAfterPrepareSurfacesRunnables -> notifyInsetsChanged
-> setInsetsChanged -> next surface-placement sends the 2nd resize.

Test: atest ActivityRecordTests# \
      testImeInsetsFrozenFlag_noDispatchVisibleInsetsWhenAppNotRequest
Change-Id: I808b37dace4a5d094e988ef77ec9f97e28327a92