Fix TH build error because of file.te

Add proc_net rules into prebuilts/api/30.0/public/file.te to fix build
errors

After applying AOSP/1468206, TH complains a build error:
Files system/sepolicy/prebuilts/api/30.0/public/file.te and
system/sepolicy/public/file.te differ

Bug: 145579144
Bug: 170265025
Test: build pass and reboot to check avc message in bugreport
Change-Id: I2085366b345c044e1b69f726809100fa43336c34
1 file changed