Bouncer update #2

Part 2 of ???. One-handed mode and user-switcher mode will be
mututally exclusive. Encapsulate one-handed logic into a separate
static class. Add logic to decide between implementations. More
support for user-switcher will be coming next.

Bug: 206825213
Test: atest KeyguardSecurityContainerTest
KeyguardSecurityContainerControllerTest

Change-Id: Iba89b62890d6e7e3252505ae38831930484ded3b