Android config updates for one handed bouncer.

This is the changes in Settings (to store the current screen side) for
the one-handed keyguard. This also checks in symbols.xml to expose the
platform config to SysUI (the lines in symbols.xml are currently in
config.xml; I'm assuming that a previous revert didn't remove them).

Bug: 170858298
Test: included.
Change-Id: I9d91a9360c821562634e83295f5e2e340f6de1d5