check-flagged-apis: make interfaces inherit from java/lang/Object

When parsing API signature files, check-flagged-apis relies on
ClassItem.superClass to get the parent class of a class or interface.
That method always returns null for interfaces.

When generating api-versions.xml, metalava marks interface classes as
inheriting from java/lang/Object:

  <class name="android/os/Parcelable" since="1">
    <extends name="java/lang/Object"/>
    [...]
  </class>

This confuses check-flagged-apis when comparing data parsed from both
sources, as the symbol signatures will be identical, but the superclass
entries differ. Work around this by explicitly marking all interfaces
as inheriting from java/lang/Object when parsing API signature files.

Bug: 334870672
Test: atest --host check-flagged-apis-test
Change-Id: Icbb8f7d4c3d4232a083289a778b347e33a0856ab

tools/check-flagged-apis/src/com/android/checkflaggedapis/CheckFlaggedApisTest.kt

@@ -120,6 +120,26 @@ class CheckFlaggedApisTest {
     assertEquals(expected, actual)
   }
 
+  @Test
+  fun testParseApiSignatureInterfacesInheritFromJavaLangObject() {
+    val apiSignature =
+        """
+          // Signature format: 2.0
+          package android {
+            @FlaggedApi("android.flag.foo") public interface Interface {
+            }
+          }
+        """
+            .trim()
+    val expected =
+        setOf(
+            Pair(
+                Symbol.createClass("android/Interface", "java/lang/Object", setOf()),
+                Flag("android.flag.foo")))
+    val actual = parseApiSignature("in-memory", apiSignature.byteInputStream())
+    assertEquals(expected, actual)
+  }
+
   @Test
   fun testParseFlagValues() {
     val expected: Map<Flag, Boolean> =

tools/check-flagged-apis/src/com/android/checkflaggedapis/Main.kt

@@ -205,7 +205,11 @@ internal fun parseApiSignature(path: String, input: InputStream): Set<Pair<Symbo
             val symbol =
                 Symbol.createClass(
                     cls.baselineElementId(),
-                    cls.superClass()?.baselineElementId(),
+                    if (cls.isInterface()) {
+                      "java/lang/Object"
+                    } else {
+                      cls.superClass()?.baselineElementId()
+                    },
                     cls.allInterfaces()
                         .map { it.baselineElementId() }
                         .filter { it != cls.baselineElementId() }