Pass pctx and ctx to NewRuleBuilder

Enable the RuleBuilder and RuleBuilderCommand methods to access
the BuilderContext by passing it to NewRuleBuilder instead of
RuleBuilder.Build.

Test: genrule_test.go
Test: rule_builder_test.go
Test: m checkbuild
Change-Id: I63e6597e19167393876dc2259d6f521363b7dabc