Change InputReader::mDevices collection type

Convert InputReader::mDevices from unordered_map of bare pointers
to unordered_map of shared_ptr's. This removes the need for manual
deletion of InputDevice instances and prepares for a future patch
which will have multiple device ids pointing to the same InputDevice.

Cherry-picked from pa/1497945.

Bug: 38511270
Test: atest inputflinger_tests libinput_tests
Change-Id: I0f3bfd96bfe5904ce1a8d96813e45f8467cee0fa