allow system_server to set ro.build.fingerprint

Some devices leave "ro.build.fingerprint" undefined at build time,
since they need to build it from the components at runtime.
See https://android.googlesource.com/platform/frameworks/base/+/5568772e8161205b86905d815783505fd3d461d8
for details.

Allow system_server to set ro.build.fingerprint

Addresses the following denial/error:

  avc:  denied  { set } for property=build.fingerprint scontext=u:r:system_server:s0 tcontext=u:object_r:default_prop:s0 tclass=property_service
  init: sys_prop: permission denied uid:1000  name:ro.build.fingerprint

Bug: 18188956
Change-Id: I98b25773904a7be3e3d2926daa82c1d08f9bcc29