Remove ndk-build from [] flac build rules

Android NDK r9 in [] is deprecated (see []

Update the ExoPlayer flac extensions to use android_jni_library.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=182017669

extensions/flac/src/main/jni/include/data_source.h

@@ -22,6 +22,7 @@
 
 class DataSource {
  public:
+  virtual ~DataSource() {}
   // Returns the number of bytes read, or -1 on failure. It's not an error if
   // this returns zero; it just means the given offset is equal to, or
   // beyond, the end of the source.