Allow AOSP source tree for compilation and fixup amcodec invoke with mpeg2

These commits change the following two things:
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**1)** Allow the use of the AOSP prebuilts/sdk and AOSP (modified) ndk to be used as the dev kits for compilation.  Checks are put in place to allow this to be conditional based on the supplied sdk and ndk during ./configure

**2)** Fixup the codec loading for DVDFactoryCodec in which mpeg2's were failing to play at full speed with no stutter with the amcodec.  Force MPEG2's to use libmpeg2 and all others fall back to the amcodec to provide better format support for dvd's.