chore(ci): Enable e2e tests

[kattrali]

Made a start. Having some issues with npm and something else which doesn't seem to be in the logs on the NDK side.

[fractalwrench]

React Native now works.

The NDK is failing due to CI using an older NDK version (13) which does not support unified headers. This results in an exception being thrown when finding the objdump executable, and means SO maps aren't generated/uploaded. From what I remember this was necessary to get NDK uploads working with AGP 3.

The fix for CI would presumably be to update our NDK fork to a later version.

[fractalwrench]

Everything passes now. I've updated the NDK scenario so that it only builds certain ABIs, and altered travis to use an NDK installed via the standard sdkmanager tool.

I also needed to make a small change to the plugin itself, from process.exitValue() to process.waitFor(), which blocks until the process has completed. On CI the objdump process was not completing in time for the SO mapping file to be uploaded, which I believe is a bug. I don't think that most users would run into this issue on a regular developer machine.