Rename IMMI#hideAllInputMethods to #hideInputMethod

This CL renames

  InputMethodManagerInternal#hideAllInputMethods()

to

  InputMethodManagerInternal#hideInputMethod()

as it is now clear that just resolving the user ID from the display ID
is sufficient in the initial concurrent multi-user IME support.

This is just a renaming as a follow up to my previouls CL [1]. There
must be no behavior change.

 [1]: I464390927a9bea86769e11a6f97f1d4796991d77
      553a5a6d

Bug: 308479256
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I7e314e3d31c6d3c3476522fba0dd88a1c9bd03dd