shell: grant access to read /proc/filesystems

Addresses the following test failure:
system/extras/tests/kernel.config/nfs_test.cpp:24: Failure
Value of: android::base::ReadFileToString("/proc/filesystems", &fs)
Actual: false
Expected: true

Denial:
avc: denied { read } for name="filesystems" dev="proc"
scontext=u:r:shell:s0 tcontext=u:object_r:proc_filesystems:s0
tclass=file

Bug: 67862327
Test: build
Change-Id: I9ada5404987cb474968afc8cb8d96137ee36c68d