Annotate ChargingStatus as a state atom.

Charging status is different from plugged status and can be used as a
state for certain metrics.

Bug: 140496920
Bug: 143495340
Bug: 145838040
Test: bit statsd_test:*
Change-Id: I1ef41191588b6d26346323203dabe488d488c131
diff --git a/cmds/statsd/src/atoms.proto b/cmds/statsd/src/atoms.proto
index 1d9f20e..4db0f91 100644
--- a/cmds/statsd/src/atoms.proto
+++ b/cmds/statsd/src/atoms.proto
@@ -1256,7 +1256,8 @@
  */
 message ChargingStateChanged {
     // State of the battery, from frameworks/base/core/proto/android/os/enums.proto.
-    optional android.os.BatteryStatusEnum state = 1;
+    optional android.os.BatteryStatusEnum state = 1
+            [(state_field_option).exclusive_state = true, (state_field_option).nested = false];
 }
 
 /**