[flexiglass] Ambient indication section.

Added as a placeholder in AOSP and a real implementation in vendor.

Test command for the vendor ambient music UI to show:
$ adb shell am broadcast -p com.android.systemui \
  -a com.google.android.ambientindication.action.AMBIENT_INDICATION_SHOW  \
  --ei com.google.android.ambientindication.extra.VERSION 1 \
  --es com.google.android.ambientindication.extra.TEXT Music \
  --el com.google.android.ambientindication.extra.TTL_MILLIS 10000

Bug: 316211368
Test: manually verified that I can see the ambient music UI in both AOD
and lockscreen after issuing the command above.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I3b61f71fe5917b04260e37b04b6faa3fd6cc382f