Add an API to dump incident report for dumpstate

Instead of just relying on the regular iteration through the system
services inside dumpstate, add another API to IIncidentManager
dedicated for dumpstate.

- It is only callable by dumpstate() (check the calling uid)
- It has the same behavior as the current call inside dump()

Advantages:
- More explicit function name, right next to takeIncidentReport will
  make it easier to keep them in sync.
- Nobody else can call it, make security easier.
- If dumpstate calls it explicitly, it can skip the 10 second timeout
- The regular dump() call should provide debugging data about
  incidentd itself, for example timestamps for the most recent N
  incident reports taken and the current state of the work directory,
  allowing us to debug incidentd itself.

Bug: 137493082
Test: Manually trigger a bug report, and verify
      /proto/incident_log.proto in the zip file.

Change-Id: I19139c765b53ede63d3beb3ea3ac40ada1aba42d