Simplify IMMS#reportPerceptibleAsync()

IMMS#mCurFocusedWindow is guaranteed to be associated with the current
IME client process.  As long as the calling process has the same
window token, checking calledFromValidUserLocked() is just redundant
and could even make it a bit unclear about what security model we are
enforcing there.

Bug: 34886274
Test: presubmit
Change-Id: Iafb2b6d82fcccb4a33eceebff3d08b96fde8dc03