Fix 2579923: Make changing lock screen method more discoverable.

This changes the organization of lock screen security settings
to make choosing an alternate unlock method more discoverable.

Instead of having to disable the old lock method to use a new
one, the user now just has one set/change option in lock settings,
with a list of method-specific setting below it.

In addition, we ask the user to confirm their old credentials
before prompting them to choose a new one, which eliminates one
source of confusion.

Also, ChooseLockGeneric now shows a UI if quality isn't specified.
Any unlock method less secure than minimum specified by
DevicePolicyManager (if active) is greyed out.

Change-Id: Iecc6f64d4d3368a583f06f8d5fe9655cc3d5bd3b