Bug 1046571 - Release wakelock on reentrant Find My Device registration.

There are two other places in which we set _registering to false: [line 312](https://github.com/guilherme-pg/gaia/blob/26d76259aa04393d480057c0b04ef0494ec25e07/apps/findmydevice/js/findmydevice.js#L312) and [line 318](https://github.com/guilherme-pg/gaia/blob/26d76259aa04393d480057c0b04ef0494ec25e07/apps/findmydevice/js/findmydevice.js#L318). In the former, we don't want to release the lock because `_requestRegistration` still needs to hold it; in the latter, we leave it to `_scheduleAlarm` to release it for us.