Skip postinstall mount check if we already completed postinstall am: 263a5400c8 am: 8e6364879e

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

Change-Id: I2cea946a4c9b3b26fbd7ee6fa3fbc058b3c3fc52
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>