[FIXED JENKINS-11083] Allow adding a suffix to generated AVD names.

This makes it possible to use the exact same emulator config in two jobs without
one job having to block waiting for the other job to finish using the emulator.

This patch comes from pull requests #9 and #33, with some additional changes to
documentation and assuring that generated AVD names are valid, despite the
suffix field being free text.
Don't consider emulator configs as equal if they have unresolved env variables.

Now we won't block builds which have build parameters which are used to set
emulator properties.  i.e. We no longer err on the side of caution to avoid
cases where multiple builds may run in parallel on the same machine with the
same variable values.