Accommodate inherited file descriptors in the subprocess utilities.

Refactored cvd::subprocess and cvd::execute into a Command and
Subprocess classes which provide the same functionality as well as
managing the lifetime of file descriptors inherited by the subprocess
and the ability to start more than one instance of the same command.

Bug: 118400788
Test: build & run locally
Change-Id: Ieb6d8873bf8294483bc8fc5449ec66e44c5fd034