isolated_app.te: Give permissions for using sdcardfs

Sdcardfs does not use a userspace daemon, so the secontext
is currently the caller's when accessing files. This can be
removed if sdcardfs is modified to change the secontext before
calling into the lower filesystem.

Bug: 32735101
Test: Run any app that falls under isolated_app.
Test: See bug for example
Change-Id: I9433aa0f14ff0d5a518249079e07f57e55b09bcf