Skip postinstall mount check if we already completed postinstall am: 263a5400c8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2336587

Change-Id: Ief2d1e9048ccb82e37e54863c2cbd942a811650e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>