Add a unique id to Perfetto trace and ANR dropbox header.

The unique id is added so that the trace and ANR can be easily linked
server side. Also moves the logging of the ANR atom before the ANR dump
takes place so that the Perfetto trace captures data closer to the point
in time when the ANR happens.

Bug: b/188122403
Test: Manual.
 1. Enabled the flag:

 adb shell device_config put perfetto_error_logger append_perfetto_uuid
 true

 2. Triggered an ANR using a test app. Logs from PerfettoTraceErrorLogger:

 adb logcat -s PerfettoTraceErrorLogger
 --------- beginning of main
 04-30 14:07:02.527 1782 12583 I PerfettoTraceErrorLogger: Pushing trace
 counter: 6c555dc6-3f46-4997-8844-706d91156600#PerfettoMonitoredCrash

 3. The UUID is present in the ANR dropbox headers:

 adb shell dumpsys dropbox data_app_anr --print | grep
 6c555dc6-3f46-4997-8844-706d91156600
 PerfettoUUID: 6c555dc6-3f46-4997-8844-706d91156600

Change-Id: If7110692cb0856e693f595e24b067ce1bc7d0a38