Add exFAT support; unify behind "sdcard_type".

We're adding support for OEMs to ship exFAT, which behaves identical
to vfat.  Some rules have been manually enumerating labels related
to these "public" volumes, so unify them all behind "sdcard_type".

Test: atest
Bug: 67822822
Change-Id: I09157fd1fc666ec5d98082c6e2cefce7c8d3ae56