Adjust default targets in RL/Module
In Ia062ff2d8b8732b0d3498c1a614bbf6a3e3a7ddb, we changed the default
targets to desktop/mobile. This broke CI for PageTriage, which has a RL
module that extends ResourceLoader\Module, and which sets the default
targets for "desktop" only.
I am not sure if leaving ResourceLoader\Module as "desktop" only was a
deliberate choice or an oversight, but it seems to be more consistent to
have it defined as desktop/mobile. This will fix the issue for
PageTriage and other extensions that extend ResourceLoader\Module and
don't have any targets explicitly defined for their RL modules.
Follows-Up: Ia062ff2d8b8732b0d3498c1a614bbf6a3e3a7ddb
Bug: T127268
Bug: T328497
Change-Id: I48ed19912610a74af66d620510c192c7ba87039a