[WebTests.Android] Lower target framework to froyo, and add test instrumentation

The test instrumentation addition allows us to run these tests in an automated fashion from command line. Simply use xbuild (or msbuild) to run the Install and Runtests targets which will respectively install, and then execute the tests on device. RunTests returns the test results to the msbuild output without needing to parse logcat. (e.g. xbuild /t:Install Xamarin.WebTests.Android.csproj; xbuild /t:RunTests Xamarin.WebTests.Android.csproj)