Android SDK: Scripts for non-interactive updates

These scripts are used by `travis-build`, see travis-ci/travis-build#226
Support for hhvm-nightly.

This support is based on phpenv/rbenv hooks so that the hhvm-nightly
package can be lazily installed to provide the most up-to-date nightly
version.

A hhvm-switcher.bash script is triggered on the exec hook and switch
betwen hhvm and hhvm-nightly packages when needed.