Update bouncer bg, button, fallback colors to match spec

Also updates couple of colors to comply with a11y contrast ratios.

Flag: com.android.systemui.bouncer_ui_revamp
Test: verified manually
Fixes: 414531398
Fixes: 414531190
Fixes: 406464411
Change-Id: I219e6dd1319237d4356c7cb597820091c5ea6dd7