init: fix variable scope issue with ExpandArgs()

ExpandArgs() was factored out of Service::Start() to clean up init,
however this introduced a bug: the scope of expanded_args ends when
ExpandArgs() returns, yet pointers to the c strings contained within
those std::strings are returned from the function.  These pointers are
invalid and have been seen to cause failures on real devices.

This change moves the execv() into ExpandArgs() and renames it
ExpandArgsAndExecv() to keep the clean separation of Service::Start()
but fix the variable scope issue.

Bug: 65303004
Test: boot fugu
Change-Id: I612128631f5b58d040bffcbc2220593ad16cd450