android: backport pty implementation from aosp/master

This is not (yet) in l-preview branch but on master
The only way to prevent duplicated includes was to
test the existence of a new constant added after
l-preview bionic source release.

Also use compatible hasmntopt() return type
(swapon/off compilation warnings)

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

Conflicts:
	Android.mk

Change-Id: I4ee6e7752eab84de4566ae86a90171b8241fcaea