allow Ninja variables in RuleBuilder API

The RuleBuilder API would not expand Ninja variables because the
variables would be written verbatim to the sandbox manifest file. This
commit allows a rule to specify that the manifest file should be written
in an un-escaped format so that Ninja variables are expanded before
writing the manifest file.

Bug: 286077158
Test: rust sandboxing topic + go test
Change-Id: I1915431f6e24d04d343dacc213c9079674ec8251