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
diff --git a/benchmarks/property_benchmark.cpp b/benchmarks/property_benchmark.cpp
index ef5f225..97eb832 100644
--- a/benchmarks/property_benchmark.cpp
+++ b/benchmarks/property_benchmark.cpp
@@ -28,8 +28,6 @@
 
 #include <benchmark/benchmark.h>
 
-extern void* __system_property_area__;
-
 // Do not exceed 512, that is about the largest number of properties
 // that can be created with the current property area size.
 #define TEST_NUM_PROPS \
@@ -53,9 +51,6 @@
       return;
     }
 
-    old_pa = __system_property_area__;
-    __system_property_area__ = NULL;
-
     pa_dirname = dirname;
     pa_filename = pa_dirname + "/__properties__";
 
@@ -111,9 +106,8 @@
     if (!valid)
       return;
 
-    __system_property_area__ = old_pa;
-
     __system_property_set_filename(PROP_FILENAME);
+    __system_property_area_init();
     unlink(pa_filename.c_str());
     rmdir(pa_dirname.c_str());
 
@@ -138,7 +132,6 @@
  private:
   std::string pa_dirname;
   std::string pa_filename;
-  void* old_pa;
 };
 
 static void BM_property_get(benchmark::State& state) {