Updated Outline.offset() docs to clarify behavior

Bug: 202063656
Test: added CTS sub-test for untested behavior
Change-Id: I092ac4d2caac97c9a3c5416aef37caad6be2b1c4
diff --git a/tests/tests/graphics/src/android/graphics/cts/OutlineTest.java b/tests/tests/graphics/src/android/graphics/cts/OutlineTest.java
index d53cec7..2e39c1b 100644
--- a/tests/tests/graphics/src/android/graphics/cts/OutlineTest.java
+++ b/tests/tests/graphics/src/android/graphics/cts/OutlineTest.java
@@ -239,5 +239,10 @@
         outline.offset(-5, -10);
         assertTrue(outline.getRect(outRect));
         assertEquals(new Rect(0, 0, 10, 10), outRect);
+
+        // Test cumulative effects
+        outline.offset(-5, -10);
+        assertTrue(outline.getRect(outRect));
+        assertEquals(new Rect(-5, -10, 5, 0), outRect);
     }
 }