Make source partitions writable during update am: 8b07db549d

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3191277

Change-Id: I5908d22a299cbd3d41dd54a4d0f8800d892e0c5d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>