Add hijack sepolicy support for rootfs devices

On older Android versions, pre-mounting selinuxfs will lead to errors,
so we have to use a different method to block init's control flow.
Since all devices that falls in this catagory must both:

1. Be Android 8.0 - 9.0
2. Have early mount fstab in its device tree

We can actually use the same FIFO trick, but this time not on selinuxfs,
but on the read-only device tree nodes in sysfs or procfs. By mocking
the fstab/compatible node in the device tree, we can block init when
it attempts to do early mount; at that point, we can then mock selinuxfs
as we normally would, successfully hijack and inject patched sepolicy.

native/jni/init/init.cpp

@@ -108,12 +108,6 @@ static int test_main(int argc, char *argv[]) {
 }
 #endif // ENABLE_TEST
 
-static int magisk_proxy_main(int argc, char *argv[]) {
-    auto init = make_unique<MagiskProxy>(argv);
-    init->start();
-    return 1;
-}
-
 int main(int argc, char *argv[]) {
     umask(0);