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.

Change-Id: Ic2c45d275179099065a10e1e664a08efea66acf1
Merge branch 'kk4.4' of https://github.com/SlimRoms/android_system_vold into kk4.4