a11y: announce theme switching and fullscreen commands

[OEvgeny]

• Add announcement for theme switching for macOS
• Make the theme switching message to start with visible content
• Move the full screen command to the UI commands and add announcement
• Refactor code to use full screen command instead of calling BrowserWindow API directly
• Update tests

VSTS-64005

[coveralls]

Coverage increased (+0.03%) to 67.661% when pulling 95ba7f35d5208c43454b47128b99a67d82401232 on OEvgeny:a11y/announce-commands into b3b5cc15ec12d5e23321fdd5d509d68343abf17a on microsoft:main.

[OEvgeny]

For the record, this removes dialog windows popping-up after fullscreen mode switching introduced in #2359 as a matter of the following reasons:

• After checking accessibility requirements, I haven't found where the requirement came from. I haven't found a Microsoft product that meets this requirement either.
• The UX is not great: users need to explicitly close the dialog after entering or leaving fullscreen mode.
• It doesn't work well on all OSes: on Mac, there is an issue toggling the mode from the menu when opening a dialog is involved.

In case the original bug gets reopened, we should bring this to the accessibility hours meeting. And even if we had to introduce the visual part, we'd need a design for this particular case.