Use cpython3 host prebuilts on most branches

But continue building them on the aosp-build-tools branch, as that's
where the prebuilts come from.

This will allow us to link against openssl for host python, even though
openssl is only available on aosp-build-tools.

Bug: 173151817
Test: ensure prebuilts are used on master, and sources on build-tools
Test: treehugger
Change-Id: I568e84fb483a0cd2f949f65cd5392d252ee8d493