Revert "Revert "vold: allow override of MAX_PARTITIONS for devices with > 4 partitions""

This reverts commit 59dcfbbc3ac8f3a43b048bbe20181b095a661253.

Conflicts:
	DirectVolume.h

Change-Id: Id56d585f10b742ffa0113f4ee6b2e69f8da84dce