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

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

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

 adb shell device_config put trace_error_logger add_error_id true

 2. Triggered a Watchdog:

 adb shell am hang --allow-restart

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

 adb shell dumpsys dropbox system_server_watchdog
 --print | grep ErrorId

 ErrorId: d27cdeee-8a6a-452b-bb0e-8cfb3eb367fa

Change-Id: I83759d905fe8626ec98595ba501e99e02e2070bf