system_properties.cpp: special case ro.* properties

Currently, reads of ro.* properties are treated differently than
writes of ro.* properties. When writing an ro.* property, we ignore
the "ro." portion of the property, and base the security decision
on the label of the remaining portion.

See https://android.googlesource.com/platform/system/core/+/e7a9e52740c952c623f7842ffa1d09b58b45e552/init/property_service.cpp
line 120-126

For example, for writing, the label associated with
"ro.build.fingerprint" comes from the /property_contexts file
entry:

  # ro.build.fingerprint is either set in /system/build.prop, or is
  # set at runtime by system_server.
  build.fingerprint       u:object_r:fingerprint_prop:s0

However, we fail to follow this same special case when sorting
properties into files. Instead, ro.build.fingerprint is assigned
u:object_r:default_prop:s0 instead of u:object_r:fingerprint_prop:s0

Ignore the "ro." portion when sorting properties into files.
This will make reads and writes of properties use the same label.

Bug: 21852512
Change-Id: Ie88ffc6b78b31fc8ddf370ae27c218546fb25a83