avcodec: add av_jni_{get,set}_android_app_ctx() helpers

This will allow users to pass the Android ApplicationContext which is mandatory
to retrieve the ContentResolver responsible to resolve/open Android content URIS.

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2024-03-xx - xxxxxxxxxx - lavc 61.3.100 - jni.h
+  Add av_jni_set_android_app_ctx() and av_jni_get_android_app_ctx().
+
 2024-03-22 - xxxxxxxxxx - lavu 59.4.100 - frame.h
   Constified the first-level pointee of av_frame_side_data_get()
   and renamed it to av_frame_side_data_get_c(). From now on,