Gallery2 app might use cleartext network traffic.

This declares that Gallery2 app might be using cleartext network
traffic (e.g., HTTP instead of HTTPS). This CL does not change the
behavior of the app. The flag added by this CL to the app's manifest
will make it easier to identify apps that use cleartext traffic in
the future.

The only reason for using cleartext network traffic is the
pre-existing public contract that this app can display externally
provided HTTP image URLs and play back externally provided HTTP and
RTSP video URLs.

Bug: 19215516
Change-Id: I6e1e8400f40ba7c280742c71af7a572b0dbad231