use adb's built-in isAppInstalled rather than one that doesn't work
add packageAndLaunchActivity support for selendroid
also convert our tests to use this by default so that it's tested
also move it to after prepareDevice so we make sure to run adb commands
with the appropriate device id etc
fix #2083, cc @bootstraponline
Merge pull request #2086 from jlipps/master

android fixes