Merge "fs_mgr: Use /proc/mounts to check if /cache is mounted" into main am: 7881073ec8

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2745619

Change-Id: Ica993eb012ed1dba6e86b7378ab96e6a4c7c2678
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>