Add feature flag to control battery app list.

If feature flag is on, then display the app list. Otherwise disable
it.

Bug: 70234293
Test: RunSettingsRoboTests
Change-Id: I1663f925abf95e0e4c1a46d98ca165303a76e979

tests/robotests/src/com/android/settings/fuelgauge/PowerUsageSummaryTest.java

@@ -83,7 +83,7 @@ import java.util.List;
 // TODO: Improve this test class so that it starts up the real activity and fragment.
 @RunWith(SettingsRobolectricTestRunner.class)
 @Config(manifest = TestConfig.MANIFEST_PATH,
-        sdk = TestConfig.SDK_VERSION_O,
+        sdk = TestConfig.SDK_VERSION,
         shadows = {
                 SettingsShadowResources.class,
                 SettingsShadowResources.SettingsShadowTheme.class,