Fix rust build in build.py

This copies the rust build result to the gn output directory to be
accessible when linking from C++. Also adds an option to use crates.io
for rust crates.

Bug: 176846220
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I9f7732878a2aced6f80f363c25aff3243d34f0d5