Add ability to choose python-for-android directory

(Edit: Following updates, the PR now creates a full system for setting buildozer options with env vars. See later comments for discussion).

This PR makes buildozer check for an environment variable, $BUILDOZER_P4A_DIR . If the variable exists, buildozer creates a symlink to this directory instead of cloning into the python-for-android github master.

Since this would cause problems with multiple apps repeatedly overwriting the p4a default distribution, the pr also changes the behaviour so that buildozer creates a distribution with the same name as app.package.name in the buildozer.spec file. This seems safe, since this already has to be something simple or build.py fails, but I'm not totally sure. The result is that any number of apps can safely share the same p4a clone as long as they have different names.

A final change as part of this is that buildozer no longer runs `git clean -dxf` before distribute.sh, in order to avoid removing the dist folders for any other apps using the same python-for-android. I'm not sure if this change is important...it seems like the clean shouldn't be necessary. Maybe I should add it as a buildozer option in the command line?