Introduce StartInputReason.SCHEDULED_CHECK_FOCUS

With this CL,

  adb shell dumpsys input_method,

tells us about whether startInputInner() is triggered by

  InputMethodManager.DelegateImpl#onScheduledCheckFocus()

or

  InputMethodManager#checkFocus().

Other than that there must be no observable behavior change.

Bug: 234882948
Test: presubmit
Change-Id: I54ebf1a4e1f749ce265756908ef52fc63f1218e5