Revert "Remove foreground user check in shell commands for IMMS"

This logically reverts my previous commit [1] with also logically
applying its next CL [2].

Reason for revert:
Not only the shell user but also any app can call 'ime' command via
IInputMethodManager.shellCommand().  Mechanically dropping
calledFromValidUser() can end up allowing unexpected cross-user
interactions (e.g. obtaining the current user's IME list from a
background user).

 [1]: I0aca79a33440f3636990ed9ef0efbbe0e4a42f77
      ca2d3b6c
 [2]: I02df0307ce2aecc77de8fb2afaa39e5ecf8f3fe2
      91e6cd0b

Bug: 34886274
Test: 'adb shell ime reset' still works
Change-Id: If8bd0ef7d95354c0485c59324b3ba3b2c258523a