do not set sdk_path in config.py if using system-provided aapt

By not setting sdk_path when /usr/bin/aapt is found, sdk_path then defaults
to $ANDROID_HOME when its used.  Since in this case, aapt will be used from
the system path, using aapt entirely ignores sdk_path.  If the user runs
`fdroid build` in this setup, sdk_path will be $ANDROID_HOME, so it should
check the env vars for it, but maybe that doesn't actually work like that
yet.

examples/config.py

@@ -9,7 +9,7 @@
 # Override the path to the Android NDK, $ANDROID_NDK by default
 # ndk_path = "/path/to/android-ndk"
 # Build tools version to be used
-build_tools = "21.1.2"
+# build_tools = "21.1.2"
 
 # Command for running Ant
 # ant = "/path/to/ant"