init: cleanup environment handling

Init keep its own copy of the environment that it uses for execve when
starting services.  This is unnecessary however as libc already has
functions that mutate the environment and the environment that init
uses is clean for starting services.  This change removes init's copy
of the environment and uses the libc functions instead.

This also makes small clean-up to the way the Service class stores
service specific environment variables.

Test: boot bullhead
Change-Id: I7c98a0b7aac9fa8f195ae33bd6a7515bb56faf78