Special case platform variant of bootstrap libs

Platform variants of libraries that set `bootstrap: true` in their
Android.bp file gets impl in Soong. This CL ports this behavior to
bp2build.

Note that even after this CL, there will be still be some other cases
where stub/impl logic does not match Soong perfectly (most notably the
platform_apis property which is propagated top-down from the parent
apex).

Test: bp2build.sh
Change-Id: I3da284ab42631d6de1c0d52e56ccbfd4e4a09f1d