Simplify IMMS#startInputOrWindowGainedFocusInternal() part 2/5

This is a series of CLs to reduce the average indentation level of
InputMethodManagerService#startInputOrWindowGainedFocusInternal().

With this CL, the IMMS lock is aquired by the caller side, which
reduces the indentation level of startInputOrWindowGainedFocus() by
one level.

This is a mechanical refactoring.  There should be no behavior change.

Bug: 34886274
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I4ccc56f376551a313fe363d48607021ed0a7933c