Relax some neverallow rules

Kernels above 4.14 have a new mmap permission. However, neverallow rules
exclude the use of mmap, even when file FDs are passable across the
vendor/non-vendor boundary. Since we allow reading / writing of passed
file descriptors, also allow the use of mmap for passed file
descriptors.

Bug: 112171217
Test: policy compiles
Change-Id: I8176f86960bdff0cf5de770809510e9df5d62db9