Eleven: Fix SettingsActivity crash

The Activity support class requires using a theme that doesn't also have
an action bar: https://developer.android.com/training/appbar/setting-up

Caused by: java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead.
	at androidx.appcompat.app.AppCompatDelegateImpl.setSupportActionBar(AppCompatDelegateImpl.java:345)
	at androidx.appcompat.app.AppCompatActivity.setSupportActionBar(AppCompatActivity.java:130)
	at org.lineageos.eleven.ui.activities.SettingsActivity.onCreate(SettingsActivity.java:53)
	at android.app.Activity.performCreate(Activity.java:7824)
	at android.app.Activity.performCreate(Activity.java:7813)
	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1306)
	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
	...

Change-Id: I9ec0d295f0c0a9cb2ebd4a9a040f1e66750a35dd