Android on Travis CI: Cache unmodified AVD to make Travis CI builds faster

[briansmith]

In #496 (comment) @briansmith wrote:

Also, would it be possible/beneficial for us to create the AVD once, have Travis CI cache it, and then make a copy of the cached AVD that we then run the tests on?

In #496 (comment), @pietro wrote:

I think I'm wrong about the AVD caching issue. I think it would be ok to cache $HOME/.android or at least $HOME/.android/avd and check if /home/travis/.android/avd/arm-18.avd exists before creating the AVD for ARMv7a.

According to Travis CI caching docs the ENV vars we export on each build is taken in consideration when creating/using the cache so non-android builds won't have the AVD cache.

[pietro]

I was playing with this and caching the AVD didn't work. I was getting a out of space error from travis and I thought that was because the AVD was too big. I looked in the latest builds and they are getting the same error just with the SDK/NDK cache. Travis docs[0] use the Android SDK as an example of things that shouldn't be cached.

I'm going to play around with caching the NDK but not the SDK.

[0] https://docs.travis-ci.com/user/caching/#Things-not-to-cache

[briansmith]

Closing this because I am not even sure it's a good idea or workable. Maybe we should be moving off of Travis-CI-specific solutions to build system issues.