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.

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Pull Request resolved: facebook#20968

Differential Revision: D9661731

Pulled By: hramos

fbshipit-source-id: ad8f2f952df2c32aea2a8d3e69c445ed27387aa6

Author: syaau

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