Store connections by token instead of by fd

The connections are currently stored by fd. If a connection is removed
via 'removeInputChannel', it is possible to re-create the same
connection and have it keyed by the same fd. When this happens, a race
condition may occur where a socket hangup on this fd would cause the
removal of a newly registered connection.

In this refactor, the connections are no longer stored by fd. The looper
interface for adding fds has two versions:
1) the old one that we are currently using, which is marked as 'do not
use'
2) the new one where a callback object is provided instead.

In this CL, we switch to the new version of the callback.

There is now also no need to store the inputchannels in a separate
structure, because we can use the connections collection that's now
keyed by token to find them.

In a future refactor, we should switch to using 'unique_ptr' for the
inputchannels. Most of the time when we are looking for an input
channel, we are actually interested in finding the corresponding
connection.

If we switch Connection to shared_ptr, we can also look into switching
LooperEventCallback to store a weak pointer to a connection instead of
storing the connection token. This should speed up the handling of
events, by avoiding a map lookup.

Test: ./reinitinput.sh. Observe that it doesnt finish after this patch
Test: atest inputflinger_tests
Bug: 182478748

Change-Id: I601f765eebfadcaeff3661a10a10c4a4f0477389