Show next alarm on ambient display

Next alarm will be visible 12h before triggering.

Test: Set alarm that will ring in 8h
Test: Set alarm that will ring in 14h
Test: Set alarm that will ring in 11:59, wait one minute
Test: atest packages/SystemUI/tests/src/com/android/systemui/keyguard/KeyguardSliceProviderTest.java
Change-Id: Icd4253771efcdf5afb4e9e52329fa410d7fd1cc1