Fix error message when Android build-tools directory is empty

Created by: makovkastar

Before this fix, when a user didn't have any Android build tools installed and tried to build an Android Buck project, he would have got the following error message: null was empty, but should have contained a subdirectory with build tools. Install them using the Android SDK Manager (/Users/username/Library/Android/sdk/tools/android). This PR provides the absolute path to the 'build-tools' directory instead of 'null': /Users/username/Library/Android/sdk/build-tools was empty, but should have contained a subdirectory with build tools. Install them using the Android SDK Manager (/Users/username/Library/Android/sdk/tools/android). It also fixes the corresponding unit test, which had the error message hardcoded.