Preserve multi-touch slot state when resetting input mappers

The existing pattern in InputReader is that when a mapper is reset, all
of its state is cleared and state of the buttons/axes is
queried through EventHub to rebuild the current state of the mapper.

This approach does not work for multi-touch devices, since there is no
way to query the current state of the contacts through the evdev
multi-touch protocol.

Since we cannot fully rebuild the current state of a multi-touch device,
we work around this limitation by never clearing the multi-touch state.

Bug: 248506674
Test: atest inputflinger_tests
Change-Id: Ic59d9fcb4e13fe262c422825c2485185004b719b