NETMGR: Allow netmgr to invoke rmnet modules

Netmgr writes to rmnet perf module parameters to control
the data path. Add policies to fix the following denials -

[   44.473586] selinux: avc:  denied  { set } for
property=persist.vendor.data.perf_ko_load pid=1346 uid=1001 gid=1001
scontext=u:r:netmgrd:s0 tcontext=u:object_r:vendor_default_prop:s0
tclass=property_service permissive=0\x0a
[  743.943376] type=1400 audit(1551998622.899:40940): avc: denied
{ read write } for comm="netmgrd" name="rmnet_perf_opt_mode"
dev="sysfs" ino=77174 scontext=u:r:netmgrd:s0
tcontext=u:object_r:sysfs:s0 tclass=file permissive=0

CRs-fixed: 2425568
Change-Id: Id24e36ff6dc25bb8c38230a147c0cce8f81a5012