Remove RemoteInputConnectionImpl#mLock, which is redudant

It is guaranteed that the following two boolean expressions have
always the same value observed outside from
RemoteInputConnectionImpl#mLock.

  A. RemoteInputConnectionImpl#mFinished
  B. RemoteInputConnectionImpl#mInputConnection != null

With that we should be able to simply merge them into an atomic
reference object

  AtomicReference<InputConnection>

without requiring RemoteInputConnectionImpl#mLock as a lock object.

This CL does so as a preparation to clean up RemoteInputConnectionImpl
for Bug 291826769.

There should be no observable behavior change, except for the fact
that

  RemoteInputConnectionImpl#dumpDebug()

no longer blocks other operations that required mLock, which is kind
of out of our original intention.

Bug: 291826769
Test: presubmit
Merged-In: I69fa2c81670f84be3dd4a808262758a803f69dfc
Change-Id: I69fa2c81670f84be3dd4a808262758a803f69dfc