[automerger skipped] Allow /postinstall files to have custom contexts am: 331e3bd90c am: b165e446cb am: 0bc81f6876 -s ours

am skip reason: skip tag Change-Id I0b8c2b2228fa08afecb64da9c276737eb9ae3631 with SHA-1 7361d27fde is already in history

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1643782

Change-Id: I3895837abeb044e3b8c1cff085def3287d2f24bd