fs_mgr: Use /proc/mounts to check if /cache is mounted

The fstab provided by the user/caller might not be the default fstab,
which might not include the /cache mount entry. We should just use the
procfs mount info to determine if /cache is currently mounted.

Bug: 300036012
Test: adb_remount test
Change-Id: I4643d0a21ae21f3513f715de424f0be1fe64ff9e