Add b autocomplete to envsetup

Note that b autocompletion isn't perfect, given that the autocompletion
script looks into the workspace root for BUILD files, instead of
out/soong/workspace. (Also, out/soong/workspace may not even be
available if bp2build has not yet been run!) However, this is a good
start to autocompletion, as bazel commands, flags, and packages
autocomplete correctly.

Fixes: 196945896
Test: Treehugger
Test: Manual verification of `b` autocompletion
Change-Id: Ia99abcfe50d195c48644c7b3455243c03d5d38f7