Let startInputOrWindowGainedFocus() take userId

This CL lets

  IInputMethodManager#startInputOrWindowGainedFocus()

take the useId as an explicit input parameter for consistency with
other IPC methods that are annotated with

  @RequiresPermission(
      value = INTERACT_ACROSS_USERS_FULL,
      conditional = true).

Doing so enables us to

 1. easily assume that INTERACT_ACROSS_USERS_FULL is necessary only
    when userId parameter is different from the calling user ID.
 2. place caller verification at the beginning of
      InputMethodManagerService#startInputOrWindowGainedFocus()
    like we do so in other Binder IPC methods.

There should be no semantic change in this CL.  This CL is purely for
better readability.

Bug: 34886274
Bug: 237316307
Test: presubmit
Change-Id: I2755fd1f2425f1c0186d46a8e4d62995c8283050