Store Connection in InputTarget and Monitor

This will replace the InputChannel. They don't both need to be shared
pointers. A lot of times, we are using InputChannel to look up the
connection. In this CL, this is simplified by using the connection
directly.

Bug: 161009324
Test: atest inputflinger_tests
Change-Id: I3fc2bbdd8c5076b5dbff5067e4fec6e45f5c3da6