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
Test: atest BugreportReceiverTest
Change-Id: I5e1802c5912f4414f1ad3b8bdaf7c7420332b9d6