Improve CI config: Define unique AVD folder and name based on other AVD values

Create 'avds' folder and export AVD and AVD_PATH env vars based on:
AVD: AVD_TARGET + AVD_TAG + AVD_ABI
AVD_PATH: TRAVIS_BUILD_DIR/avds + AVD (name)

This is part of a fix for the same issue defined here
https://android-review.googlesource.com/#/c/113019/

A problem occurred when trying to create a new avd with the same name
as that previously used by another, renamed avd.
https://code.google.com/p/android/issues/detail?id=78677