Handle type mapping in signature files

This is a hot fix for the broken build.

Test: Build cf_x86_phone-userdebug
Change-Id: Id43930212cfa2ac0e7261ecae3f6298e7d141e8d

src/main/java/com/android/tools/metalava/model/text/TextClassItem.kt

@@ -254,6 +254,10 @@ open class TextClassItem(
     override fun qualifiedName(): String = qualifiedName
     override fun toString(): String = qualifiedName()
 
+    override fun mapTypeVariables(target: ClassItem): Map<String, String> {
+        return emptyMap()
+    }
+
     companion object {
         fun createClassStub(codebase: TextCodebase, name: String): TextClassItem =
             createStub(codebase, name, isInterface = false)