Returns immediately if the bugreport file already exists

There's a case that BugreportProgressService is invoked twice quickly,
and both services create the same bugreport file name. The later one
may delete current running bugreport file in its clean function,
when it detects another bugreport is running.

Bug: 174314124
Bug: 175287931
Test: atest BugreportReceiverTest
Change-Id: I5e1802c5912f4414f1ad3b8bdaf7c7420332b9d6
Merged-In: I5e1802c5912f4414f1ad3b8bdaf7c7420332b9d6