Remove @UnsupportedAppUsage from InputMethodManager#mServedInputConnectionWrapper

As already mentioned in Bug 115609023 and confirmed in Bug 194308519,
we are mostly confident that this @UnsupportedAppUsage was a false
positive.

Bug: 115609023
Fix: 194308519
Test: presubmit
Change-Id: Ied59e96c3a22da5eb11c2f3dfd5cc54734e87110