Add Android.mk with host targets.

Upstream's cmake build system generates some include files during
configure time needed by the build. This patch includes those files in
a new android_include/ directory that expossed as part of the library
include directories. The headers were generated with the
generate_includes.sh script so they can be updated when needed.

This patch includes an Android.mk with the host targets.

Bug: 24327902
Test: mma external/libdivsufsort

Change-Id: Iee5274bc2ce22bd6ac66f4dd9b0c196536d52a3a