Dumpstate aidl: add cancelBugreport

This allows adding a cancel method to the bugreporting API. Note that it
is implemented on the java side.

Also removed unused args from onFinished.

BUG: 111441001
Test: adb shell /data/nativetest64/dumpstate_test/dumpstate_test

Change-Id: I6a3a0d652c89dc157de4a4056ccddc40e5d6e549