Check that return types are assignable instead of exact.

We now check that the new return type is assignable to the old
return type instead of just comparing the types as strings when
checking the consistency of two APIs.

Fixes:
https://android-review.googlesource.com/#/c/58843/

Change-Id: I6a18a76e5296d58c05f1bb59341cdaa820f0c540
diff --git a/test/api/changed-assignable-return-1.xml b/test/api/changed-assignable-return-1.xml
new file mode 100644
index 0000000..8166bbd
--- /dev/null
+++ b/test/api/changed-assignable-return-1.xml
@@ -0,0 +1,9 @@
+<api>
+<package name="test" >
+<class name="A" extends="java.lang.Object" abstract="false" static="false" final="false" deprecated="not deprecated" visibility="public" />
+<class name="B" extends="test.A" abstract="false" static="false" final="false" deprecated="not deprecated" visibility="public">
+<method name="me" return="test.A" abstract="false" native="false" synchronized="false" static="false" final="false" deprecated="not deprecated" visibility="public" />
+</class>
+</package>
+</api>
+