Fix wrong bootanimation.zip from mkdir .../part{0..2}

For some reason the mkdir command:
mkdir -p $ANDROID_PRODUCT_OUT/obj/BOOTANIMATION/bootanimation/part{0..2}
creates a directory part{0..2} instead of three separate directories.
Non-fatal build errors and a 386 byte bootanimation.zip file result.
This explicitly creates each directory.

Change-Id: Ia6ae0e4f64521992de8cc34a376af3eaac5c8819