Improve Travis CI Android Builds

[pietro]

This PR improves the Android situation on Travis CI. It caches some of the Android things we need to download for every build and adds AArch64 builds. It fixes both #486 and the issue I wrote about on #518.

[ghost]

Can't find anything malicious in the changes.

[briansmith]

Thanks a ton for this!

I rebased these changes and pushed them (otherwise unmodified) to master:

52c645b Don't install unnecessary packages on Travis CI Android builds.
b7073f4 Cache Android NDK, tools and platform tools on Travis CI.
06da374 Add support to install different Android system images on Travis CI.
759f1cc Define Android API level needed for AArch64.
cd011b3 Add AArch64 to Travis CI builds.
ffe7e97 Use travis_wait on all Android release builds on Travis CI.
4388505 Add AArch64 build info to README.

[pietro]

Cool! Hopefully next week I'll have time to debug the 32-bit ARM issues when using clang.

[briansmith]

@pietro After this change some builds are failing with "No space left on device": https://travis-ci.org/briansmith/ring/jobs/240986948

[coveralls]

Coverage decreased (-0.07%) to 95.859% when pulling b99358d on pietro:travis_android into 2db042d on briansmith:master.

[briansmith]

@pietro I had to revert these changes due to the build failures. Not sure what we can do. Maybe we can delete intermediate inputs after the input has been used and is no longer needed. And/or maybe when we need to rebuild a cached file, we should delete all the prevously- (stale) cached files before attempting the rebuild?

[briansmith]

I'm going to close this PR in favor of switching to cross for building and testing Android. Thanks for all your contributions here!