Move property setting from libcutils to bionic.

All the other property stuff is already here.  Property setting was
only in libcutils previously to leverage a utility function / constant
or two.

Unfortunately in the process of fixing a race condition we would've
had to do break abstraction boundaries and put some libc-internal
details into libcutils so instead of that we'll just move this
into bionic.

Along with Iee1ca9b7, this now passes:

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

Bug: 3511230
Change-Id: I110b653a58f312fbe069dca59892a877ae9bc911
diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index 4fdc944..85915b2 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -46,6 +46,10 @@
 */
 int __system_property_get(const char *name, char *value);
 
+/* Set a system property by name.
+**/
+int __system_property_set(const char *key, const char *value);
+
 /* Return a pointer to the system property named name, if it
 ** exists, or NULL if there is no such property.  Use 
 ** __system_property_read() to obtain the string value from