bp2build: codegen singleton lists on one line.

This CL refactors the bp2build code generator to pretty print lists with
a single element on one line, instead of taking up three lines, which
can make BUILD files unnecessarily long. A single line singleton list is
also more commonly used in BUILD files.

Test: TH
Change-Id: Ic9e44741bbb070c8f45925466b9ccdd0608498b2