main/postmarketos-ui-phosh: set appropriate gschema for pinephone keyboard symbols

fixes #1587 (closed)

RightAlt is the default Alternative Characters Key, but the symbol layer on the pinephone keyboard accessory is not triggered by the right alt key on a fresh install. It must be manually set to something else, then set back to 'Right Alt' to work. Setting it in the ui package (in the same place some other gsettings are set) should enable it to work without manually changing the setting. This should not adversely effect other devices since this is supposed to be the default setting.

I installed v22.06 fresh on an SD card, installed this modified package, rebooted, and confirmed that symbols are working without ever opening gnome-control-center to change the settings.