fs_mgr: correct support for ro.build.system_root_image

Failed to convert fstab entries from "/" to "/system" when setting
up directory tree.  Introduce an internal fs_mgr_mount_point to
simplify conversion.

Test: manual
Bug: 109821005
Change-Id: Iadc1e967b92702cf01b6522f8f13b2cf3685f2af