Only start TaskPersister once.

Because ActivityManagerService.systemReady() is reentrant we could
restore tasks and start the TaskPersister more than one time. This
fix limits operations on TaskPersister to one time only.

Fixes bug 15256579.

Change-Id: I6bf2c26b37acdfd9b15a6f277966966b743d03b6
1 file changed