Add default values for preferences (#8584)

Defaults have been added for all three preferences. The default values
added are both falsey, so this shouldn't result in any functional
change. This was done to help make this preferences more easily
discoverable.

app/scripts/controllers/preferences.js

@@ -52,6 +52,8 @@ export default class PreferencesController {
       lostIdentities: {},
       forgottenPassword: false,
       preferences: {
+        autoLockTimeLimit: undefined,
+        showFiatInTestnets: false,
         useNativeCurrencyAsPrimaryCurrency: true,
       },
       completedOnboarding: false,

ui/app/ducks/metamask/metamask.js

@@ -39,8 +39,9 @@ export default function reduceMetamask (state = {}, action) {
     welcomeScreenSeen: false,
     currentLocale: '',
     preferences: {
-      useNativeCurrencyAsPrimaryCurrency: true,
+      autoLockTimeLimit: undefined,
       showFiatInTestnets: false,
+      useNativeCurrencyAsPrimaryCurrency: true,
     },
     firstTimeFlowType: null,
     completedOnboarding: false,