Refine getNativeFileDescriptor

Factor out the isAtLeastR method, allow the "T" codename in addition to
"S", and add some comments.

Also remove usage of __ANDROID_API_S__ as it is not yet available in all
branches.

Bug: 166414751
Test: built, installed, wifi and tethering working
Change-Id: I473e6b224b670c18a85fa33d6ec3b4cc2873ce94