Use aconfig for Shade Migration flags

Merge MIGRATE_NSSL and MIGRATE_KEYGUARD_STATUS_VIEW to a single
aconfig flag.

Bug: 278054201
Test: atest SystemUITests
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl DEVELOPMENT
Change-Id: I153504a9052799c368246430eb274898a9a02888