Reverted CarServiceHelperService.runAsync() to use its own thread.

It was using SystemServerInitThreadPool, which could crash the system server
if the user pre-creation task took too long.

Test: atest CarHelperServiceTest
Test: manual verification by using SystemClock.sleep(30_000)

Fixes: 157263778

Change-Id: I052521a9a7256efe7ebf590d56c17a2e22a43af8