Android video source & control (Prototype)

Use the following in your stbt.conf:

    [global]
    source_pipeline = filesrc location=android-screenshots ! pngparse ! pngdec ! videoconvert
    control = android

Requires GStreamer 1.0 (pngparse/pngdec on GStreamer 0.10 didn't support
multiple consecutive PNGs read from the same file).