Use prebuilt otatools for mixed build

Enable mixed_build script to use prebuilt otatools for packaging
image files. If we have otatools.zip file, use '-t' option to use
the prebuilt tools instead of building them in mixed branches.

Bug: 112732863
Test: run mixed_build with otatools using the command below:
 $ development/vndk/tools/build_mixed -v 9 \
   -m development/vndk/tools/modify_system_img.sh \
   -p out/prebuilt_cached/artifacts/system/vbmeta.img \
   -t out/prebuilt_cached/artifacts/system/otatools.zip \
   out/prebuilt_cached/artifacts/system \
   out/prebuilt_cached/artifacts/taimen \
   dist \
   out/prebuilt_cached/artifacts/system/checkvintf

Change-Id: I7891a2bad2f4e471baffb987149a39bbbd57f26e