Save build.prop files when trimming OTA/target_files am: f3f85a1c1a

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2123560

Change-Id: I009eb62f19569c9254c7d4401391b1a3c646d3a3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>