jest: Expand coverage to include Android-only codepaths.

This means that our Jest tests will take ~twice as long as they took
before. We'll soon add an option to `tools/test` to run the Jest
tests for just one platform, which we can use locally to speed
things up, just without getting as much coverage as we'd otherwise
get. See discussion at
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.60jest-expo.2Fios.60.20and.20.60jest-expo.2Fandroid.60/near/1169139.

jest.config.js

@@ -50,5 +50,8 @@ const common = {
 
 module.exports = {
   // See https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
-  projects: [{ ...common, displayName: 'ios', preset: 'jest-expo/ios' }],
+  projects: [
+    { ...common, displayName: 'ios', preset: 'jest-expo/ios' },
+    { ...common, displayName: 'android', preset: 'jest-expo/android' },
+  ],
 };

jest/jestSetup.js

@@ -39,16 +39,13 @@ polyfillGlobal('URLSearchParams', () => URLSearchParams);
 // [1] https://github.com/facebook/react-native/issues/26579#issuecomment-535244001
 // [2] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3781.20RN.20v0.2E61.20upgrade/near/931219
 jest.mock('react-native', () => {
-  ReactNative.NativeModules.ZLPConstants =
-    // Currently only available on iOS. We don't bother
-    // conditionalizing on the platform here; if we want to mock
-    // Platform.OS, we'll likely want to do so per-test. In that
-    // scenario, we'll need to figure out how to recompute this mock
-    // with Platform.OS set as desired.
-    {
+  if (ReactNative.Platform.OS === 'ios') {
+    ReactNative.NativeModules.ZLPConstants = {
       resourceURL:
         'file:///private/var/containers/Bundle/Application/4DCD4D2B-F745-4C70-AD74-8E5F690CF593/ZulipMobile.app/',
     };
+  }
+
   return ReactNative;
 });