Use consistent spinners for loading screens (#2611)

* Use consistent spinners for loading screens

* Consolidate into LoadingScreen

src/view/com/util/LoadingScreen.tsx

@@ -0,0 +1,14 @@
+import React from 'react'
+import {ActivityIndicator, View} from 'react-native'
+import {s} from 'lib/styles'
+import {CenteredView} from './Views'
+
+export function LoadingScreen() {
+  return (
+    <CenteredView>
+      <View style={s.p20}>
+        <ActivityIndicator size="large" />
+      </View>
+    </CenteredView>
+  )
+}