Bring back some physical keyboard settings

These preferences were originally in AOSP, but they were then removed
and we brought them back with 572f90bd
("Settings: Add toggles for several CM additions"). So they are not
our addition as commit 5aa2f34b
("Settings: Add toggles for several CM additions") implies and they
are currently dummy switches. Bring back the code needed to handle
these preferences.

Please note that these preferences simply control whether the current
on-screen keypad can or cannot process hard keyboard events.

BUGBASH-761

Change-Id: I4e90e009a998fadfcbd48b07aa9ad3164a01cb1a