Don't hold all static keyboard layouts in memory

[spoenemann]

Follow-up of #4724: Currently the browser module browser-keyboard-layout-provider loads all available static keyboard layouts. I expect the list of statically available layouts to grow, so at some point the memory footprint might become too large. We should improve this by holding only the active layout in memory. Furthermore, we should avoid to include all layouts in the bundle.js, so we might move them to a backend service.

[spoenemann]

This is not possible after #5063 because we need to evaluate all layouts in the browser when the user presses a key.

The total size of all layouts contributed in #5088 is 184 KB (json files).