ANDROID: Fix modules.order for external modules.

With MO properly set, modules.order from external modules
contains simple relative paths below MO, instead of prefixed
with KBUILD_EXTMOD. Hence, the substitution no longer worked
for installing the module.

Fix the substitution so that we simply prepend INSTALL_MOD_DIR
directly.

Bug: 402231876
Change-Id: Id3b014475a8f5562f3664ee7ed1ae50bc8df67af
Signed-off-by: HONG Yifan <elsk@google.com>