Mount multiple DSU partitions when present.

There might be partitions like product or system_ext in addition
to the system partition. Those partitions can contain dependencies
required by the system so we need to make the init to recognize
these partitions and mount them accordingly.

Bug: 140092208
Test: gsi_tool install & reboot
Change-Id: Ie5ef063292c3656e79cce9f887f23a8faa1ba2be