Make ImeTrackerService pending check asynchronous

This modifies hasPendingImeVisibilityRequests from the ImeTrackerService
from a blocking binder call that returns a boolean, to an asynchronous
binder call that reports back to the caller using an AndroidFuture. This
method is only used in tests, which can now replace the existing
PollingCheck on a blocking call with a single call that signals as soon
as its done. This change allows marking the IImeTracker interface as a
whole oneway.

Flag: EXEMPT bugfix
Test: atest InputMethodStatsTest
  MultiDisplayImeTests#testFallbackImmMaintainsParameters
Bug: 342111149
Change-Id: I51bdfec2965d8b01f181d6505e3df9dbb95e7190