Inline ImeFocusController#onInteractive()

Now that

  ImeFocusController#onInteractive()

is simple enough to be inlined, which allows us to also remove

  InputMethodManagerDelegate#isCurrentRootView().

This is mechanical refactoring.  There must be no behavior change.

Bug: 234882948
Test: presubmit
Change-Id: I4f001e7cef6e274a481cd1dc9ce314f3b19313dd