Support sandboxing droiddoc and droidstubs with args properties

args properties can access arbitrary files with $(location) expansions,
so they need to pass them through RuleBuilderCommand.PathsForInputs
to produce a path inside the sandbox.  Extract the arg expansion out
of collectDeps into a new expandArgs method that takes the
RuleBuilderCommand.

Test: TestDroidstubsSandbox
Change-Id: I9022d17bf3cb64c97b2008c4c1b733bf48edca95