Fix a race in system properties.

This now passes:

$ adb shell am instrument -w -e class android.os.SystemPropertiesTest \
  com.android.frameworks.coretests.systemproperties/android.test.InstrumentationTestRunner

(which tests setting and re-reading back system properties in a loop)

This patch depends on I110b653a which moves property setting
from libcutils into bionic.

Bug: 3511230
Change-Id: Iee1ca9b7defd6060510a01f40fbe63ba4c96eb8d