Android: findAvd() return serialNumber instead of bool

So that the function can be reused by waitForAvd.

Change-Id: I0b7a86a13bf6a5f5b16d1d1cab362e7200a9a26f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Android: Rewrite waitForAvd()

Now takes a QFutureInterface to enable canceling and beautify the code
by splitting it up.

Change-Id: Ifedf19c3ad1a37a9b7cb6b7db8ec799ceebc5392
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>