Add a new "virtual disk" feature.

It's extremely difficult to test storage related logic on devices
that don't have physical SD card slots.  So to support better
debugging and testing, add a new "virtual disk" feature which mounts
a 512MB file through loop device.

Also move ParcelFileDescriptor.open() over to using Os.open() so
that it gets StrictMode treatment.

Bug: 34903607
Test: builds, boots, virtual disk works
Change-Id: I072a3a412cfcc8a2a3472919b7273a1ed794fd98