BootReceiver: Added an event log after the dropbox file copy is successful.

The log can be used to test if LAST KMSG or other items are copied
to dropbox successfully, especially in user builds without root
privilege.

BUG: 69685635

Test: manually verified the desired log from bugreport on user
and userdebug builds.

Change-Id: Ie6033bf04c7f79fc596761ab751aa5fcea2c1130