IosCompliantAppleAuthButton types: Limit what can get passed for `sty…

…le`.

Partly in the spirit of 8b30fa9, but also because this helps us
avoid lots of mystifying error reports from Flow when we take the
upgrade to Flow v0.122.0, with the React Native v0.63 upgrade
(zulip#4245).

See discussion of those errors [1], and Greg's conclusion that Flow
might have been a bit more helpful than it was here. Ah, well.

[1] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Flow.20errors.20in.20.23M4318/near/1075420

src/start/IosCompliantAppleAuthButton/index.js

@@ -1,10 +1,11 @@
 /* @flow strict-local */
 import React, { PureComponent } from 'react';
 import { View } from 'react-native';
-import type { ViewStyleProp } from 'react-native/Libraries/StyleSheet/StyleSheet';
+import type { ViewStyle } from 'react-native/Libraries/StyleSheet/StyleSheet';
 import * as AppleAuthentication from 'expo-apple-authentication';
 import { connect } from '../../react-redux';
 
+import type { SubsetProperties } from '../../generics';
 import Custom from './Custom';
 import type { ThemeName } from '../../reduxTypes';
 import type { Dispatch } from '../../types';
@@ -15,7 +16,15 @@ type SelectorProps = $ReadOnly<{|
 |}>;
 
 type Props = $ReadOnly<{|
-  style?: ViewStyleProp,
+  // See `ZulipButton`'s `style` prop, where a comment discusses this
+  // idea.
+  /* eslint-disable flowtype/generic-spacing */
+  style?: SubsetProperties<
+    ViewStyle,
+    {|
+      marginTop?: mixed,
+    |},
+  >,
   onPress: () => void | Promise<void>,
 
   dispatch: Dispatch,