c2_e2e_test: update E2eTestActivity theme

This updates the activity's theme so it can play videos fullscreen
without any extra ui.

Bug: 166403789
Test: manually launch C2E2ETest and make it fullscreen

Change-Id: Ida2b50405781cd11a45284ecd93e6630154deba5

tests/c2_e2e_test/AndroidManifest.xml

@@ -19,7 +19,8 @@
         android:allowBackup="false"
         android:label="@string/app_name">
         <activity android:name=".E2eTestActivity"
-                  android:launchMode="singleTop">
+                  android:launchMode="singleTop"
+                  android:theme="@android:style/Theme.NoTitleBar.Fullscreen">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />