New API to instantiate a MediaExtractor given a URI and optional MIME type.

Also DataSources now must provide a method initCheck()
and DataSource::reat_at has been renamed to readAt to conform to
standard API naming guidelines.
diff --git a/media/libstagefright/DataSource.cpp b/media/libstagefright/DataSource.cpp
index 933c2f6..210b2f6 100644
--- a/media/libstagefright/DataSource.cpp
+++ b/media/libstagefright/DataSource.cpp
@@ -28,7 +28,7 @@
     *x = 0;
 
     uint8_t byte[2];
-    if (read_at(offset, byte, 2) != 2) {
+    if (readAt(offset, byte, 2) != 2) {
         return false;
     }