Fix NPE in BootReceiver

BootReceiver is depend on DropboxManagerService.
If native crash occured before DropboxManagerService start,
BootReceiver.addTombstoneToDropBox will occus NPE due to DropboxManager is null.
So it need delay watch the tombstone file.

Change-Id: I188f18663ac6966d82b97b77e8fed72238adf683
1 file changed