Subprocess: Close all unused file descriptors.

This patch extends Subprocess::ExecFlags() method with a list of
file descriptors that should be kept open on the child process and
mapped to a pipe() in the parent. The remaining file descriptors
will be closed.

Bug: 27880754
TEST=Added unittests for this functionality.

(cherry picked from commit e384bb2e)

Change-Id: I06fe529c58e179315e5a740d03c43c3b2d852526