Add uptimeNanos to SystemClock

Test: system/core/libutils/SystemClock_test.cpp

Bug: 162791194
Change-Id: I0e3ea2004146eb7442c89bb35a9f3a2171d9c4b8
diff --git a/libutils/SystemClock.cpp b/libutils/SystemClock.cpp
index 73ec1be..9c71141 100644
--- a/libutils/SystemClock.cpp
+++ b/libutils/SystemClock.cpp
@@ -39,8 +39,15 @@
  */
 int64_t uptimeMillis()
 {
-    int64_t when = systemTime(SYSTEM_TIME_MONOTONIC);
-    return (int64_t) nanoseconds_to_milliseconds(when);
+    return nanoseconds_to_milliseconds(uptimeNanos());
+}
+
+/*
+ * public static native long uptimeNanos();
+ */
+int64_t uptimeNanos()
+{
+    return systemTime(SYSTEM_TIME_MONOTONIC);
 }
 
 /*