Use fstab in the source build

When generating incremental OTAs, we should use the fstab in the source
build instead of the target one. Similarly for recovery_mount_options.

Bug: 22011892
Change-Id: Ia02584c71af7d47835b1f62e7daa46f27940a545
(cherry picked from commit bebd3cfbf934beb18b73a4d4e98b98c2c0a1d6fe)
2 files changed