interceptor: refactor argv_vector() out of execute_execve

This allows later reuse when adding support for intercepting
posix_spawn().

Bug: 209401200
Signed-off-by: Matthias Maennich <maennich@google.com>
Change-Id: I035d4e2a2e2539c073a9c8bcf919872137fb2d28
1 file changed