[webOS] xkb keymap improvements

[sundermann]

Description

Maps the back button to BROWSER_BACK which seems to fit more than ESCAPE. Also map invalid key presses to UNKNOWN so they don't spam the log.

Motivation and context

How has this been tested?

What is the effect on users?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[jenkins4kodi]

I've made some formatting changes to meet the current code style. The diffs are available in the following links:

• PR23180-0001-4ce3863f4a-webOS-xkbcommon-Map-back-button-as.diff
• PR23180-0002-ca5fcf77b8-webOS-xkbcommon-Map-webOS-invalid-.diff
• PR23180.diff

For more information please see our current code style guidelines.