Deliver key events to the system if no accessibility service handles them.

We have APIs that allow an accessibility service to filter key events. The
service has to declare the capability to toggle event filtering in its
manifest and then it can set a flag to toggle the feature at runtime. The
problem was that even if no accessibility service was handling key events
these events were not fed back to the input system.

This change adds a new feature flag in the accessibility input filter that
is set only if at least one service can and wants to filter key events. If
the feature flag is set then the filter will deliver events to services and
,if they are not handled, to the system. This change also cleaned the logic
for updating the input filter.

bug:8713422

Change-Id: I4bc0c1348676569d1b76e9024708d1ed43ceb26a