Let IMM directly call checkFocusInternal() when possible

With this CL, the code paths from IMM#checkFocus() will be drastically
simplified as follows.

 * Before
    InputMethodManager#checkFocus()
     -> ImeFocusController#checkFocus()
      -> InputMethodManager.DelegateImpl#checkFocus()
       -> InputMethodManager#checkFocusInternal()

 * After
    InputMethodManager#checkFocus()
     -> InputMethodManager#checkFocusInternal()

In the end,

  InputMethodManager.DelegateImpl#checkFocus()

is guaranteed to be called only from ViewRootImpl.

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

Bug: 234882948
Test: presubmit
Change-Id: Ie1c7525ee46db11ccd593c6ed0ac17b4ece41953