Bug 863664 - [email] Use ArrayBuffer instead of Uint8Array. rs=squib

land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/185
https://github.com/mozilla-b2g/gaia-email-libs-and-more/commit/ef73f19dfd6aa12f8e86d1174d4c76480509a02a