do not merge. Fix a race in system properties.

Backport Iee1ca9b7defd6

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: I145e642bd13079677017bb64e361e176fc73b810