Bouncer User Switcher - Use flags

The user switcher will be enabled per device, mostly for larger form
factors. Add a flag based on a config value, that can be overlayed or
overriden with the flag app. Also correct some layout issues and
positioning for pin/pw/pattern for larger form factors.

Bug: 206825213
Test: atest KeyguardSecurityContainerTest
Change-Id: Ib8c059bd5739162d3ffe7a1c86fcd757e87701bf