Save environment snapshot and use at fork/exec

Some applications may inadvertently or maliciously set of environment
variables such as LD_LIBRARY_PATH before spawning subprocesses.
To make this more difficult, save the environment at the time the
runtime starts and use the saved copy anytime Exec is called.

BUG: 30160149
TEST: make test-art-{host,target}

Change-Id: I887b78bdb21ab20855636a96da14a74c767bbfef
(cherry picked from commit d106d9f8)