Add new DockStateChanged atom.

Android Metrics Design Review: eldar/963674278.

Bug: 226553050
Test: N/A
Change-Id: I8715435afad4eb82446efe2b4e51ae9b6601cdd2
Merged-In: I8715435afad4eb82446efe2b4e51ae9b6601cdd2
(cherry picked from commit 1701a3f74a3b25e4c05229856c29c68fe7511977)