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

Summary: This reverts commit 59dcfbbc3ac8f3a43b048bbe20181b095a661253.

Test Plan: win

Reviewers: randall.rushing, synergy

Reviewed By: synergy

CC: jriordan001

Differential Revision: http://review.codefirex.com/D352

Change-Id: I792b44315befcb25cde47c490fc64a8b2103896a