Make DownloadBase public

It's the base class for some public classes, so it needs to be public
as well according to the CTS rules.

Bug: 2537352
Change-Id: Ie2f8141d56907e1d0f4a3a040204b7b14d1fd79a

api/current.xml

@@ -88413,7 +88413,7 @@
  static="true"
  final="false"
  deprecated="not deprecated"
- visibility="private"
+ visibility="public"
 >
 <method name="startDownloadByUri"
  return="long"

core/java/android/net/Downloads.java

@@ -469,7 +469,7 @@ public final class Downloads {
     /**
      * Base class with common functionality for the various download classes
      */
-    private static class DownloadBase {
+    public static class DownloadBase {
         /** @hide */
         DownloadBase() {}