Generate FakeFeatureFlagsImpl in test mode

Before FeatureFlagsImpl will be used as a fake for testing. This change
adds new class FakeFeatureFlagsImpl. The FeatureFlagsImpl will keep the
same as production. FakeFeatureFlagsImpl can be used as fake for
testing.

FakeFeatureFlagsImpl, and FeatureFlagsImpl will be generated in both
test and prod mode. In test mode FeatureFlagsImpl will just be a stub,
and in prod mode FakeFeatureFlagsImpl will just be a stub.

Bug: 280833463
Test: atest aconfig.test
Change-Id: I11c1e716a9ea00d55600e5e9d5fb6442420762e6

tools/aconfig/templates/FakeFeatureFlagsImpl.java.template

@@ -0,0 +1,59 @@
+package {package_name};
+{{ if is_test_mode }}
+import static java.util.stream.Collectors.toMap;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.stream.Stream;
+
+public class FakeFeatureFlagsImpl implements FeatureFlags \{
+{{ for item in class_elements}}
+    @Override
+    public boolean {item.method_name}() \{
+        return getFlag(Flags.FLAG_{item.flag_name_constant_suffix});
+    }
+{{ endfor}}
+    public void setFlag(String flagName, boolean value) \{
+        if (!this.mFlagMap.containsKey(flagName)) \{
+            throw new IllegalArgumentException("no such flag" + flagName);
+        }
+        this.mFlagMap.put(flagName, value);
+    }
+
+    public void resetAll() \{
+        for (Map.Entry entry : mFlagMap.entrySet()) \{
+            entry.setValue(null);
+        }
+    }
+
+    private boolean getFlag(String flagName) \{
+        Boolean value = this.mFlagMap.get(flagName);
+        if (value == null) \{
+            throw new IllegalArgumentException(flagName + " is not set");
+        }
+        return value;
+    }
+
+    private HashMap<String, Boolean> mFlagMap = Stream.of(
+            {{-for item in class_elements}}
+            Flags.FLAG_{item.flag_name_constant_suffix}{{ if not @last }},{{ endif }}
+            {{ -endfor }}
+        )
+        .collect(
+            HashMap::new,
+            (map, elem) -> map.put(elem, null),
+            HashMap::putAll
+        );
+}
+{{ else }}
+{#- Generate only stub if in prod mode #}
+public class FakeFeatureFlagsImpl implements FeatureFlags \{
+{{ for item in class_elements}}
+    @Override
+    public boolean {item.method_name}() \{
+        throw new UnsupportedOperationException(
+            "Method is not implemented.");
+    }
+{{ endfor}}
+}
+{{ endif }}