Address @hides in ImageDecoder

Bug: 152322291
Test: make

Once ImageDecoder is in a mainline module, the system can no longer
access @hidden methods (unless they're @SystemApi), so this
removes/documents some. The ones in Source are not used anywhere outside
ImageDecoder AFAICT, so they are unneeded. Document setOutPaddingRect,
which needs to be public/@hide so it can be accessed by the drawable
package, which will also be in the module.

Change-Id: I5cd9c0553e8d27d6679cbc431fcadc96d818687d

graphics/java/android/graphics/ImageDecoder.java

@@ -195,14 +195,11 @@ public final class ImageDecoder implements AutoCloseable {
     public static abstract class Source {
         private Source() {}
 
-        /* @hide */
         @Nullable
         Resources getResources() { return null; }
 
-        /* @hide */
         int getDensity() { return Bitmap.DENSITY_NONE; }
 
-        /* @hide */
         final int computeDstDensity() {
             Resources res = getResources();
             if (res == null) {
@@ -212,7 +209,6 @@ public final class ImageDecoder implements AutoCloseable {
             return res.getDisplayMetrics().densityDpi;
         }
 
-        /* @hide */
         @NonNull
         abstract ImageDecoder createImageDecoder(boolean preferAnimation) throws IOException;
     };
@@ -1413,6 +1409,8 @@ public final class ImageDecoder implements AutoCloseable {
      *  {@link OnHeaderDecodedListener#onHeaderDecoded onHeaderDecoded}.</p>
      *
      *  @hide
+     *  Must be public for access from android.graphics.drawable,
+     *  but must not be called from outside the UI module.
      */
     public void setOutPaddingRect(@NonNull Rect outPadding) {
         mOutPaddingRect = outPadding;