C-ify DataSource

Add a C API for DataSource/Base, and a helper so extractors
can continue using a C++ API.

Bug: 111407253
Test: build, boot, play some files

Change-Id: I1c8b2990e17d18eee53c9abf7ebef2ced0e8b7fd
diff --git a/media/extractors/mp3/VBRISeeker.h b/media/extractors/mp3/VBRISeeker.h
index 9213f6e..507899c 100644
--- a/media/extractors/mp3/VBRISeeker.h
+++ b/media/extractors/mp3/VBRISeeker.h
@@ -24,11 +24,11 @@
 
 namespace android {
 
-class DataSourceBase;
+class DataSourceHelper;
 
 struct VBRISeeker : public MP3Seeker {
     static VBRISeeker *CreateFromSource(
-            DataSourceBase *source, off64_t post_id3_pos);
+            DataSourceHelper *source, off64_t post_id3_pos);
 
     virtual bool getDuration(int64_t *durationUs);
     virtual bool getOffsetForTime(int64_t *timeUs, off64_t *pos);