Rename ImeFocusController#checkFocus() to #onScheduledCheckFocus()

Now that

  ImeFocusController#checkFocus()

is called only from ViewRootImpl to handle MSG_CHECK_FOCUS.

Hence, it would make much more sense to rename it to

  ImeFocusController#onScheduledCheckFocus().

with also inlining hard-coded parameters.

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

Bug: 234882948
Test: presubmit
Change-Id: I354acacb2d35d378ee7bc140ac7bc6311bfec156