fs_mgr: overlayfs: empty system partitions excluded.

Beside using the 1% threshold for determining that a filesystem
should be an overlayfs candidate, do so also if the free space
is below 8MB.  This will cover nearly empty filesystems that can
not be right-sized below the smallest ext4 filesystem size.

Adjust the test to cover the new cases.

Should test all system partitions, including new /system_ext and
any others that get added in the future.

Fix instance of dir that were supposed to be die.  Using || die
does not work as expected within a function, so use if ! <cond>.

Test: adb-remount-test.sh
Bug: 139471424
Change-Id: If3e4cac5dce7bee998727052130eaf362e9e2fb5