HACK: Need to startup dhcpd for non-system bluetooth stack

Add TODO comments to remove start/stopReverseTethering
Return error code with exit() in child process in startReverseTethering
when executing dhcpcd fails.
This is needed so we can get BT certification.
The cleanup is tracked in stopship b/6820232

Change-Id: I98a1c8aedfa042b1824de7d8c69f2600c9bfcf29