Remove @UnsupportedAppUsage from IInputConnectionWrapper#mInputConnection

As already mentioned in Bug 115609023 and confirmed in Bug 194297014,
we are mostly confident that this @UnsupportedAppUsage was a false
positive.

Bug: 115609023
Fix: 194297014
Test: presubmit
Change-Id: Ie70099c2f5f926ce64324998a61eb54649e553f9