[fs_mgr] fix -Wreorder-init-list

C++20 will require members in a designated initializer to be in order
unlike C99.

Bug: 139945549
Test: mm
Change-Id: I8804f7dd5cba1035ac7a2979a47b661d722f664a
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>