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

Also removes mentions of libnativehelper test (it no longer uses
__system_property_area__) and removes useless "extern" declaration
(actual use was removed long ago).

Test: refactoring CL, existsing tests still pass

BUG=21852512
BUG=34114501

Change-Id: I2223cab2fcb671ea180ad4470a7aba5c9cd20bd8