Add engines back to build

The new keystore work requires OpenSSL engines to be enabled again.

This adds back the Android.mk rules, tweaks the android-config.mk to
build engine support, and adds in some essential files that were deleted
during the subsequent merges since engine support was removed
(crypto/engine/tb_asnmth.c and crypto/engine/tb_pkmeth.c)

Change-Id: I7b8190d21f50c1dd1ed12bbd0961b6e543c18ecf