vold: Allow ASEC containers on an external volume when internal is emulated

Thanks to Pierre-Hugues Husson for the idea in this patch! The initial
implementation called createBindMounts() on all mount points regardless
of whether or not it was the SD card.

Conflicts:
	VolumeManager.cpp

Change-Id: I817ec018d5edf666dd341c74aab95bb404eed061