vold: Allow ASEC containers on external SD when internal is emulated

WARNING: This an initial implementation for Android 4.4 Kit Kat.
It may or may not work properly.

* This modifies Volume::mountVol() and Volume::unmountVol to call
  mountAsecExternal() to set up .android_secure/ regardless if the
  volume is primary or not. On devices where emulated internal
  storage is used, the emulated storage is primary. Without this
  patch, /data/secure/asec/ and the tmpfs over .android_secure/
  won't be mounted.

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