init: always expand args in subcontext

Currently init expands properties in arguments only when those
commands are run in a subcontext.  This creates a hole where
properties that should not be accessible from a given subcontext of
init can be accessed when running a command in the main init
executable (for example `start`).

This change creates a callback in subcontext init that simply expands
and returns arguments back to the main init process, to ensure that
only those properties that a subcontext can access get expanded.

Bug: 62875318
Test: boot bullhead, new unit tests
Change-Id: I2850009e70da877c08e4cc83350c727b0ea98796