Use interface for $(location) values in genrules

Use an interface instead of a string to retrieve the value for
$(location) or $(locations) expansions in genrules to allow
delaying the evaluation until the RuleBuilderCommand is available.
This allows using helpers like RuleBuilderCommand.PathForInputs
to properly rewrite the values for sandboxing.

Also remove the standalone SboxPathFor* methods that don't operate
on a specific RuleBuilderCommand that are now unnecessary.

Test: genrule_test.go
Change-Id: I8bb2647332ef118204a216cead23d062517e2b8c