Fix issue with canvas usage

In order to avoid DOM exceptions, icons should not be draw to a canvas
until their image has completed loading.
Generate expected HTML elements in unit tests

The statusbar logic expects the provided element to define a
`getContext` method, so a `canvas` element is the appropriate choice.
Extend dialer Utils mock for unit tests
Initialize new stub for every dialer ussd test

Tests modify the state of the global `navigator.mozL10n` object, so this
object should be re-initialized before each test.