Use shared_ptr for EventEntry

Instead of manual refcounting, use std::shared_ptr for EventEntry. This
will make code less error-prone.

We could probably keep unique pointers in mInboundQueue. The only
problem is "mNextUnblockedEvent".

One idea to work around this issue is to prune the queue using event
id's instead. Another idea is to prune the queue right on the spot when
queueing. That might not be OK because it would happen on the
inputreader thread.

Bug: 142581626
Bug: 167946924
Test: presubmit
Change-Id: I6626017180da2d202e2e3acc7a8200107abde3d7