[applets/systemtray] Tablet mode friendliness

Two commits, each with a video to show what it does:

    [applets/systemtray] Use "scale with panel" setting when in tablet mode
    
    This setting is more appropriate when in tablet mode, as it makes the
    System Tray icons scale up to fill the panel's height/width. When tablet
    mode exits, it returns to the default icon size setting (unless the user
    already set to always be in "scale with panel" mode.
    
    CCBUG: 439922

processed-20210805_125138

    [applets/systemtray] Scale expander arrow with "scale with panel" setting
    
    This makes the expander arrow match the scale of all the other icons
    when using the "Scale with panel width" setting. Otherwise it looks too
    small and is hard to use in tablet mode, if you're using that setting for
    increased touch-friendliness.
    
    BUG: 439922
    FIXED-IN: 5.23

simplescreenrecorder-2021-08-05_12.13.42

cc @teams/vdg