Add an exported flag in manifest

With c4e90ac, the android:exported flag has to be explicitly defined
if an intent filter is defined.

Bug: 171447284
Test: install IntentPlayground
Change-Id: I4f86ceeef70e7572707f548f0583011a0f0823ad

samples/IntentPlayground/AndroidManifest.xml

@@ -24,7 +24,7 @@
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
         android:theme="@style/AppTheme">
-        <activity android:name=".RegularActivity">
+        <activity android:name=".RegularActivity" android:exported="true">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />