minijail: generate libconstants source file in Android.mk

Add a rule to generate libconstants.c in minijail's Android.mk.

While there, add the -Wno-sign-compare C flag so that minijail will
build without errors.

BUG: 24680644
TEST: "mm" in the minijail repo succeeds.
Change-Id: Ifa89e704686aea52a619532d5e626d97969284cf
1 file changed