Add @hide IMM#getEnabledInputMethodSubtypeListAsUser()

This CL adds an @hide method

  InputMethodManager#getEnabledInputMethodSubtypeListAsUser()

so that bundled apps with INTERACT_ACROSS_USERS_FULL permission such
as the Settings app can query about different users' IME subtypes.

Bug: 275106096
Test: presubmit
Change-Id: If5f2781b85e503cf059ef42102c09d99d6b9cb6b