Extract IMM internal flags into StartInputFlags

This is a mechanical refactoring like we did to split
InputMethodClient for Bug 118040692.

So-called "controlFlags" in InputMethodManager (IMM) was originally
introduced for IMM#startInput()/windowGainedFocus() [1] to carry
additional client information then reused when we unify startInput()
and windowGainedFocus() into startInputOrWindowGainedFocus() [2].

This CL mechanically moves the location where those flags are defined,
from InputMethodManager.java to a newly created file
StartInputFlags.java.

This is still just a mechanical refacotring / renaming.  There should
be no user-visible behavior change.

 [1]: Icb58bef75ef4bf9979f3e2ba88cea20db2e2c3fb
      7663d80f
 [2]: I56934f18e30d90fcdf77bcbb0c35a92a5feb1b82
      05c25f8a

Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Test: atest FrameworksCoreTests:com.android.internal.inputmethod.InputMethodDebugTest
Change-Id: If5a4810dece852edcff0d1119463711249bb7ef2