Add AndroidTest_Coverage.xml

The NetworkStackCoverageTests target needs an AndroidTest.xml, because
it is run as part of mts.

Using an AndroidTest.xml file would cause other tests in the same
directory to use it and get the wrong configuration; instead add
AndroidTest_Coverage.xml and explicitly specify the configuration.

Test: m mts && mts-tradefed mts-network-coverage
Change-Id: Ic578ed7da852d6a5f39937d2ea9de4e364262ce8