Stop using __system_property_area__

Apparently that "backdoor" is no longer needed - the proper way is
to reinitialize properties:
  https://android-review.googlesource.com/#/c/181794/24/tests/system_properties_test.cpp

Test: refactoring CL, existsing tests still pass

BUG=21852512

Change-Id: I2fe193d65a0e6804bfc15726d9b973a97b378fef