added gradle adb tasks for apk installs and test runs

Also you can use the following parameters with all adb* tasks:
*  -DadbPath=<path to adb>
*  -DdeviceId=<android-serial>
For the adbRunTests task:
*  -DnoDeviceTests=true    -> Not tests with the @ Device annotation are run
*  -DonlyDeviceTests=true  -> Only tests with the @ Device annotation are run
*  -DtestClass=<class>
*  -DtestPackage=<package>

I've created a jenkins jobs where these new tasks are already used instead of shell commands:
https://jenkins.catrob.at/view/All-Categories/view/Experimental/job/Catroid-AdbTasks-TestJob