android/build.py: use os.path.abspath() to build mpd_path

Fixes a bug that occurs when runing "build.py" from inside the
"android" directory.

android/build.py

@@ -23,7 +23,7 @@ if not os.path.isdir(ndk_path):
     sys.exit(1)
 
 # the path to the MPD sources
-mpd_path = os.path.dirname(os.path.dirname(sys.argv[0])) or '.'
+mpd_path = os.path.abspath(os.path.join(os.path.dirname(sys.argv[0]) or '.', '..'))
 
 # output directories
 lib_path = os.path.abspath('lib')