Add TODO about IMM#getInputMethodWindowVisibleHeight()

InputMethodManager#getInputMethodWindowVisibleHeight() has been kind
of unintentionally exposed to all the IME client processes and we know
some apps are already relying on this hidden API via reflection, which
is bad especially because it would be getting harder and harder to
keep maintaining the semantics of this not-well-defined hidden API
once we start supporting features like multi display. Thust this CL
adds TODO comments there to warn people who are going to touch this
method.

This CL does nothing except for adding comments.  Hence there should
be no behavior change.

Bug: 113914148
Test: compile
Change-Id: Id2084530bdb82deef226d8207d2f311d2e78b28d