Make source partitions writable during update am: 8b07db549d am: df004c998a

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

Change-Id: Ibe969deed00a55fced30a725fe6876f21ea15838
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>