Unmount product partition in GSI

This is a workaround solution. It let GSI to unmount product
partition when booting. The patch avoid the problem that

- share UID between between system and product partition
- the product partition override the content under /system/product

Bug: 119391482
Test: Boot aosp_arm64-userdebug on a blueline device with Android P
Change-Id: I48808fcc1ca527a23eb1446b97176b08268b500e