sepolicy: Allow apps to read ashmem fds from system_server

Kernel commit 8a2af06415ef0fc922162503dd18da0d9be7771f (ashmem: switch
to ->read_iter) switched ashmem from using __vfs_read to vfs_iter_read
to read the backing shmem file.  Prior to this, reading from an ashmem
fd that was passed between processes didn't hit any permission checks;
now SELinux checks that the receiver can read from the creator's file
context.

Some apps receive buffers through ashmem from system_server, e.g., the
settings app reads battery stats from system_server through ashmem when
an app details page is opened.  Restore this ability by giving apps read
access to system_server_tmpfs.  system_server is still responsible for
creating and passing across the ashmem buffers, so this doesn't give
apps the ability to read anything system_server isn't willing to give
them.

Bug: 112987536
Bug: 111381531
Test: atest android.appsecurity.cts.PermissionsHostTest on kernel 4.14
Change-Id: Ice5e25f55bc409e91ad7e8c7ea8b28ae213191a3