Bug 1027950 - Correctly initialize find my device settings panel. r=arthurcc

    * Do not attach handlers inside onerror callback.
    * Disable fmd login button until watch fires.
    * Add tests.
Merge pull request #20774 from 6a68/bug-1027950-settings-fmd-onerror-handlers

Bug 1027950 - [Settings][Find My Device] Do not attach handlers inside onerror callback