Add tests for routeSocketDataToOutput.

Also fixes the problem where it could through IndexOutOfBoundsException
instead of more natuarl EOFExcpetion in case there is not enough data in
the chunk.

Bug: 38081946
Bug: 37983441
Test: runtest -p com.android.server.backup frameworks-services
Change-Id: Icac575a76b752922b9c2d9cc0b3cad50f4273241