Permission Check For DPM Get IME API

Require the caller of DPM.getPermittedInputMethodsForCurrentUser() to
hold the MANAGE_USERS permission.  The only callers should be settings
apps which already hold this permission.

Bug: 62343414
Test: Manage IME list in the Settings app
Test: com.google.android.gts.devicepolicy.DeviceOwnerTest#testPermitInputMethods
Change-Id: I0d162f8f51d16e403a950ee5d942502c2cf20181