Add process start time to ProcessMemoryState atom

Also, add the start time to non-additive in statsd.

Bug: 118249210
Test: manually verified values are included in a report
Change-Id: Ib91d4d0f416a1cbd298f4a010e8264a3e8f0ee16
diff --git a/cmds/statsd/src/atoms.proto b/cmds/statsd/src/atoms.proto
index ca049b0..5620184 100644
--- a/cmds/statsd/src/atoms.proto
+++ b/cmds/statsd/src/atoms.proto
@@ -2399,6 +2399,10 @@
     // Peak RSS usage of the process. Value is read from the VmHWM field in /proc/PID/status or
     // from memory.max_usage_in_bytes under /dev/memcg if the device uses per-app memory cgroups.
     optional int64 rss_high_watermark_in_bytes = 9;
+
+    // Elapsed real time when the process started.
+    // Value is read from /proc/PID/stat, field 22. 0 if read from per-app memory cgroups.
+    optional int64 start_time_nanos = 10;
 }
 
 /*