TranslationProvider [nfc]: Annotate TranslationContext.

Despite passing `undefined` as the `defaultValue` argument to
`React.createContext`, consumers will never see `undefined` as the
value for `this.context` or `useContext(TranslationContext)`.

That is, as long as `TranslationContext.Provider` is above such
consumers in the component hierarchy. From React's doc on
`createContext` [1],

"""
The `defaultValue` argument is **only** used when a component does
not have a matching Provider above it in the tree. This can be
helpful for testing components in isolation without wrapping them.
"""

Adding this annotation will satisfy the new version of Flow
(0.122.0) that we'll get along with the React Native v0.63 upgrade
(zulip#4245) soon.

[1] https://reactjs.org/docs/context.html#reactcreatecontext

src/boot/TranslationProvider.js

@@ -1,5 +1,5 @@
 /* @flow strict-local */
-import React, { PureComponent } from 'react';
+import React, { PureComponent, type Context } from 'react';
 import type { ComponentType, ElementConfig, Node as React$Node } from 'react';
 import { Text } from 'react-native';
 import { IntlProvider, IntlContext } from 'react-intl';
@@ -11,7 +11,7 @@ import { getSettings } from '../selectors';
 import messages from '../i18n/messages';
 
 // $FlowFixMe could put a well-typed mock value here, to help write tests
-export const TranslationContext = React.createContext(undefined);
+export const TranslationContext: Context<GetText> = React.createContext(undefined);
 
 /**
  * Provide `_` to the wrapped component, passing other props through.