Give system_server / system_app ability to write some properties

Allow writing to persist.sys and debug.

This addresses the following denials (which are actually being enforced):

<4>[  131.700473] avc:  denied  { set } for property=debug.force_rtl scontext=u:r:system_server:s0 tcontext=u:object_r:shell_prop:s0 tclass=property_service
<3>[  131.700625] init: sys_prop: permission denied uid:1000  name:debug.force_rtl
<4>[  132.630062] avc:  denied  { set } for property=persist.sys.dalvik.vm.lib scontext=u:r:system_app:s0 tcontext=u:object_r:system_prop:s0 tclass=property_service
<3>[  132.630184] init: sys_prop: permission denied uid:1000  name:persist.sys.dalvik.vm.lib

Change-Id: I5d114c0d963bf393f49f1bf13d1ed84137fbcca6