Ship ARM64 builds

[cpeterso]

We need to publish 64-bit builds to the Google Play Store before Google's August 1 deadline requiring 64-bit support (for apps that include native code).

https://android-developers.googleblog.com/2019/01/get-your-apps-ready-for-64-bit.html

It looks like Fenix is only shipping 32-bit ARMv7 and x86 builds:

fenix/.taskcluster.yml

Lines 92 to 93 in 19dd468

	--apk arm/release/app-arm-release-unsigned.apk \
	--apk x86/release/app-x86-release-unsigned.apk \

Fennec Nightly started shipping ARM64 builds on 2019-01-15. The Reference Browser already started shipping ARM64 Nightly builds on 2019-01-17 (issue mozilla-mobile/reference-browser#367).

GeckoView publishes ARM64 builds in Maven, but Gecko itself is missing some ARM64 optimizations (SpiderMonkey's Ion JIT) so ARM64 performance might be slower than ARMv7. The ARM64 GeckoView builds should be adequate for Nightly testers, but we should not ship to Beta or Prod users until Gecko's Ion JIT supports ARM64: Firefox bug 1520261 and these blocker bugs listed on this Mobile ARM64 wiki. ARM64's ETA is 2019-05-13 for GeckoView and Fennec 68 Beta.

┆Issue is synchronized with this Jira Task