Add a unique build number to every build

SDK-readable via android.os.Build.INCREMENTAL

Change-Id: If9885adebe12a56b2b2e353285721be695bc411b

Fix exporting the unique build identifier

Change-Id: Ie58e981b7e83a4351d137dfd8995dce7d365d950

Fix incremental build number on Darwin

Darwin does not include sha1sum, but it should have openssl.

Change-Id: If2b685c1478f9486bed21dfba0c655d9e02db72b

Use openssl instead of sha1sum

Change-Id: Iab8a74d4af6646104cb23312853f811a38bdec0d