Unhide InputMethodService#requestShowSelf()

This is a follow up CL to a recent CL [1], which aimed to move several
APIs only for InputMethodService from InputMethodManager to
InputMethodService.

This CL removes InputMethodService#hideSoftInputFromInputMethod(),
which is exactly the same as InputMethodService#requestHideSelf() that
is already available as a public API for IME developers.

This CL also virtually renames
InputMethodService#showSoftInputFromInputMethod() to
InputMethodService#requestShowSelf(), which has existed as a
private method but not been exposed to IME developers yet.

 [1]: I3163f3cbe557c85103ca287bee0874a3b4194032
      d8d03a8e

Bug: 70282603
Test: atest CtsInputMethodTestCases
Change-Id: If6a786c5774805d041ea9672ef2721e4a38df7fc