More robust display ID mismatch detection in IMM

This is another follow up CL to my previous CL [1] that enabled
per-display InputMethodManager (IMM) instance (Bug 115893206).

What we learned in Bug 118341760 is that for some apps
view.getContext() may return a ContextWrapper subclass whose
Context.getDisplayId() and Context.getSystemService() are not
consistent with each other.  Although this is considered to be a bug
of such a ContextWrapper subclass, application developers may not be
aware of the issue when such a problematic ContextWrapper came from
libraries that app developers happened to, or had to, depend on.

The key idea of this CL is that

  view.getViewRootImpl().getDisplayId()

would be a more robust source of display ID than

  view.getContext().getDisplayId()

for most of cases, because to which ViewRootImpl a given View belongs
is already a strong signal about to which display the View belongs.

As far as I've tested locally, this approach seems to be more
promising and is expected to give us better app compatibility in
multi-display scenarios.

 [1]: I78ad7cccb9586474c83f7e2f90c0bcabb221c47b
      4052a10f

Bug: 118252837
Fix: 118341760
Test: atest ActivityManagerMultiDisplayTests
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:android.view.inputmethod.InputMethodManagerTest
Change-Id: If47e48bc8176657bf4bb882146d2affdfe457c90