ImageDecoder: Add getters. Rename setAsAlphaMask

Bug: 73788928
Test: I1606cbb4e71579160ffaef12c1ed738fad882cd1

This will allow Kotlin developers to treat the setters as properties.

Part of the motivation for the rename is that "getAsAlphaMask" sounds
like it returns an alpha mask itself. Rename both to
_etDecodeAsAlphaMask.

Change-Id: I8f9b04f8381840490b662c3bcd37a95442af8110
diff --git a/api/removed.txt b/api/removed.txt
index b8e2b98..a5370f4 100644
--- a/api/removed.txt
+++ b/api/removed.txt
@@ -152,6 +152,11 @@
     method public deprecated boolean clipRegion(android.graphics.Region);
   }
 
+  public final class ImageDecoder implements java.lang.AutoCloseable {
+    method public deprecated boolean getAsAlphaMask();
+    method public deprecated android.graphics.ImageDecoder setAsAlphaMask(boolean);
+  }
+
   public deprecated class LayerRasterizer extends android.graphics.Rasterizer {
     ctor public LayerRasterizer();
     method public void addLayer(android.graphics.Paint, float, float);