Use the round launcher icon available since 7.1 (facebook#20968)

Summary:
The default react-native template has ic_launcher_round icons on the resources but has not been used.

Thank you for sending the PR! We appreciate you spending the time to work on these changes.
Help us understand your motivation by explaining why you decided to make this change.

If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.
Pull Request resolved: facebook#20968

Differential Revision: D9661731

Pulled By: hramos

fbshipit-source-id: ad8f2f952df2c32aea2a8d3e69c445ed27387aa6

local-cli/templates/HelloWorld/android/app/src/main/AndroidManifest.xml

@@ -8,6 +8,7 @@
       android:name=".MainApplication"
       android:label="@string/app_name"
       android:icon="@mipmap/ic_launcher"
+      android:roundIcon="@mipmap/ic_launcher_round"
       android:allowBackup="false"
       android:theme="@style/AppTheme">
       <activity