[fs][devmgr] Avoid using inequality operators on ordinals

Although there is still heavy reliance on ordinals as constants
outside generated FIDL code, this patch removes the filesystem
dependence on relative ordering of FIDL ordinals by instead
utilizing the "try_dispatch" family of functions.

Rather than routing to the appropriate dispatch function, the FIDL
handler will now "try_dispatch" to all possible interfaces,
checking against ERR_NOT_SUPPORTED until something else is returned
or the list is exhausted.

This is less efficient, but acts as a workaround for FTP-20.

ZX-3063 #done
Test: CQ

Change-Id: Iff1736757417e2b44765ae0083fc6d16022102fe