Move genrule on top of RuleBuilder

In preparation for more complicated sandboxing support in sbox, use
a single implementation of the sbox sandboxing by moving genrule to
use RuleBuilder's sbox support instead of creating an sbox rule
directly.

Also move genrule's input list hash support into RuleBuilder.

Test: genrule_test.go
Test: rule_builder_test.go
Change-Id: I292184d02743c7e6887ebbcd232ba565db2ab0cc