Added support for vendor partition to adb

Remount will now remount the vendor partition as well, if it exists.
Sync will also allow you to sync vendor, and will include it by
default if it exists.

Change-Id: Iea1e8212f445e96233438a8d8a9d3266bf3d6557
Signed-off-by: Daniel Rosenberg <drosen@google.com>