Optimize IME Input Target Update

In [1] we started reporting the Binder (token) of the
startInputOrWindowGainedFocus window through InputMethodService
setImeWindowStatus, sending this value to WindowManagerService, but not
using it there.

However, this was quite slow, and in many cases was reported after the
IME started showing, so in [2] we modified this further to report the
token through InputMethodService dispatchStartInput, into
reportStartInputAsync, still without using the value in WindowManager.

Later in [3] we started using the reported token to update the IME
Control Target (and the IME Input Target). However, there is no need for
this flow to take a round trip through InputMethodService, as there are
no checks there, and we don't need to synchronize with the IME. Instead,
all the checks are done inside the InputMethodManagerService.

This optimizes the IME Input Target update flow by removing the round
trip into InputMethodService, aiming to reduce some flickers that could
be caused by slower updates.

  [1]: I54fb1faf513883a54293a756e4c9d2ae9453d778
  [2]: I032b91ce722a16b17518a5d88015c574d7d3e51b
  [3]: I9e8984b7e5aa989a53ece9e2576393f795b9ef94

Flag: android.view.inputmethod.optimize_ime_input_target_update
Bug: 427397699
Test: atest CtsInputMethodTestCases
Change-Id: I8e25eb09c89b40b52f28d7e18124e43ecb8dc99e