Make sure to call back View#onCheckIsTextEditor() on the UI thread

Calling back View#onCheckIsTextEditor() from a non-UI thread is
a well known type of bugs that we have met in the past [1].

This time it turns out that it can still happen when
InputMethodManager#isActive() gets called on a background thread.

This CL should address such a remaining case.

 [1]: I098df41f935438bf1889fb030c7a3e7a0645a168
      3f733cc7

Bug: 286016109
Test: atest CtsInputMethodTestCases:FocusHandlingTest#testOnCheckIsTextEditorRunOnUIThreadWithInputMethodManagerIsActive
Change-Id: I015603c1b33ecdf4a4f5d5be5d85756a3b2b1615
(cherry picked from commit b9eecc6b)