Create aapt2_artifacts dist goal for aapt2

Building aapt2_artifacts will output aapt2 and aapt2-protos.zip to
DIST_DIR. If BUILD_HOST_static is not specified, aapt2.exe will also
be built.

Bug: 147476179
Test: m -j dist DIST_DIR=~/dist_test aapt2_artifacts
Test: m -j dist DIST_DIR=~/dist_test BUILD_HOST_static=1 \
      aapt2_artifacts
Change-Id: I9a0ee769f8c6579f1e01c04e42c62ebe850ba401