Fix DATE_MODIFIED assertion fail in image media store test.

Getting the last modified from the created test file would be more accurate
than getting it from current system time.

Change-Id: I3d7834c6e728f49b055666ad985930ab37729bd6
diff --git a/tests/tests/provider/src/android/provider/cts/MediaStore_Images_MediaTest.java b/tests/tests/provider/src/android/provider/cts/MediaStore_Images_MediaTest.java
index 8e8d650..a8b14bc 100644
--- a/tests/tests/provider/src/android/provider/cts/MediaStore_Images_MediaTest.java
+++ b/tests/tests/provider/src/android/provider/cts/MediaStore_Images_MediaTest.java
@@ -273,7 +273,8 @@
         values.put(Media.TITLE, "testimage");
         long dateAdded = System.currentTimeMillis() / 1000;
         values.put(Media.DATE_ADDED, dateAdded);
-        long dateModified = System.currentTimeMillis() / 1000;
+        File externalFile = new File(externalPath);
+        long dateModified = externalFile.lastModified() / 1000;
         values.put(Media.DATE_MODIFIED, dateModified);
 
         // insert