Renames customization content provider (3/7).

KeyguardQuickAffordanceProvider has started to be used for more than
just lock screen shortcuts. This collection of CLs updates its name,
authority, and table schema to become more generically about "system UI
customization".

Fix: 262879277
Test: manually verified Settings > Display > Lock screen shows the
"shortcuts" item
Test: manually verified that the Wallpaper picker properly renders the
preview and can change the quick affordances
Test: manually verified that system UI displays the right lock screen
shortcuts

Change-Id: Ib9c73b5171e0bffa6f4e56904e5d672495420243