Simplify IMMS#startVrInputMethodNoCheck a bit

This is a follow up CL to a recent CL [1] that added VR-IME support.

Currently, IMMS#startVrInputMethodNoCheck() internally calls
IMMS#setInputMethodEnabled(), which enforces the caller user ID check
and caller permission check for WRITE_SECURE_SETTINGS.

This appears to make sense for IMMS#startVrInputMethodNoCheck(), but
in reallity IMMS#startVrInputMethodNoCheck() always gets called on
non-binder system server thread hence those checks will never fail.

Note that such a call path is fine and not a security issue.  What this
CL tries to address is just replace IMMS#setInputMethodEnabledLocked()
to make it clear that IMMS allows such an operation for callers that
have RESTRICTED_VR_ACCESS is intentional.

To summarize, this CL just removes redundant operations and there
should be no visible behavior change.

 [1]: I1db7981b5198e7e203d4578cae7e5b6d20037d0d
      89a6c48a

Bug: 63037786
Test: compile
Change-Id: Iec43177bd698de6efa453d64ceafec3164538151