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=b/30160149
TEST=make test-art-{host,target}-gtest-utils_test

Change-Id: Ice0097bf0de30db3d4ead124caf93316f39e3df3