*   Android demo: Makefile build functionality added to build.gradle to fully
    support building TensorFlow demo in Android on Windows.
*   Android demo: read MultiBox priors from txt file rather than protobuf.
*   Added colocation constraints to `StagingArea`.
*   `sparse_matmul_op` reenabled for Android builds.
*   Restrict weights rank to be the same as the broadcast target, to avoid
    ambiguity on broadcast rules.