Wait for persisting data into the dropbox if system_server crashes

On process crashes, we'll kill the faulty process and also log
the event into the dropbox asynchronously. However, if that process
is the system_server itself, it might be killed before the event
is persisted into the dropbox successfully. So now we'll wait for
a while for the dropbox.

Bug: 295976585
Test: Manual - induce a system_server crash and check dropbox.
Change-Id: I0f3155d5ccfa039b9c5da6502ff5e9588e19acea