Allow startBugreport to run async

The startBugreport call uses a listener interface to
provide return status for bug report generation;
however, for convenience the method would previously
block until the underlying service was successfully
launched. But, the underlying call is slow, so it would
block the caller for hundreds of millis or for up to
30 seconds. This patch allows the startBugreport() call
to run asynchronously instead of blocking the caller.

Bug: 180123623
Test: atest BugreportManagerTest; (cts, and unit test)
Test: manual test
Change-Id: Ibbc0de76a1c12c979772222208704ad0e466b1a5