Ensure Autofill is off by default (duckduckgo#1707)

Task/Issue URL: https://app.asana.com/0/0/1204558657522442/f

Description:

The code which was merged into production for the Autofill release had Autofill set to "enabled by default”. This should not be the case. It should be off by default. This PR addresses that.

DuckDuckGo/AppUserDefaults.swift

@@ -187,7 +187,7 @@ public class AppUserDefaults: AppSettings {
             // In future, we'll use setAutofillCredentialsEnabledAutomaticallyIfNecessary() here to automatically turn on autofill for people
             // That haven't seen the save prompt before.
             // For now, whilst internal testing is still happening, it's still set to default to be enabled
-            return userDefaults?.object(forKey: Keys.autofillCredentialsEnabled) as? Bool ?? true
+            return userDefaults?.object(forKey: Keys.autofillCredentialsEnabled) as? Bool ?? false
         }
 
         set {

DuckDuckGoTests/AppUserDefaultsTests.swift

@@ -88,6 +88,11 @@ class AppUserDefaultsTests: XCTestCase {
 
         XCTAssertEqual(appUserDefaults.currentThemeName, .systemDefault)
     }
+
+    func testDefaultAutofillStateIsFalse() {
+        let appUserDefaults = AppUserDefaults(groupName: testGroupName)
+        XCTAssertFalse(appUserDefaults.autofillCredentialsEnabled)
+    }
 
     /*
      These tests aren't required until we make autofill default to off, and then enable turning it on automatically