Prevent InputMethodManager call into DelegateImpl (2/N)

InputMethodManager#DelegateImpl is a delegation for the
ImeFocusController to call InputMethodManager.

Wrap the DelegateImpl#startInput implemetation into a
dedicated method so InputMethodManger don't need to
rely on the delegation class.

Bug: 243778447
Test: atest CtsInputMethodTestCases
Change-Id: I0f98dfa5f08d51f6e9627c84f0921161374ad0e3