Create /dev/selinux folder if it was not created by first-stage-init

The /dev/selinux folder is normally created by first-stage-init
https://cs.android.com/android/platform/superproject/+/master:system/core/init/first_stage_init.cpp;l=299-300;drc=07c86bace1e7a7cc4dd975f10b077d21c3743749

However, in some cases the first-stage-init comes from a GKI prebuilt
boot.img and doess not create the required folder (see for example
b/217677967), resulting in bugs like b/244793900.

I modified the selinux.cpp code to create the /dev/selinux folder if it
does not exist already, as a safety measure.

To verify these changes for b/244793900, follow gpaste/4922166775644160

Bug: 243923977
Test: atest SeamendcHostTest, manual verification
Change-Id: I8fe798643b1aeab2b4caac837055348febf70b94