Fix code to extract platform target number.

The previous code worked only if the targetSdkVersion attribute was in a single
line. Now we use matchstr function to extract the version even if the attribute
is between other attributes in one line.
Add uninstall function.
Update doc with new uninstall function
Improve install function.

Instead of calling ant on each device to install we now call ant once to
compile the APK file and then use adb to intsall the APK file on each device.
This reduces the installation time on several devices.

As bonus we also check for errors during installation and can warn the user
that the installation failed on a device.