Custom message in AoD

Optional message that lives under the clock and also
in AoD. Implemented using a slice.

Test: launch sysui, look at the date in AoD
Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/keyguard/KeyguardSliceProviderTest.java
Change-Id: I9dbc827bd48c6a3d1b25b0a481ffaecc0dd37f25