Add support for Android TV

[chenxiaolong]

This required a bit of hackery in webview_bridge.js to make Syncthing's web UI navigable via arrow keys. There are a few main issues:

• Many web UI elements don't have a visually distinct focused state, so we inject some CSS to add a dotted border around the focused element.
• The dropdown menus have a keydown handler coming from Bootstrap that hijacks the up/down arrow keys, making it difficult or impossible to leave the dropdown menus using the browser's builtin spatial navigation.
• The <input type="number"> elements have default browser behavior of incrementing or decrementing the number using the arrow keys. There are no spatial navigation APIs available yet, so we can't just preventDefault() these events. Instead, we have a horrible hack to make the field immutable for 100ms while the default handler runs.

There are also some limitations in functionality due to Android TV:

• Importing, exporting, and saving logs will never work because DocumentsUI doesn't exist.
• The persistent notification isn't visible because notifications for third party apps isn't supported at all. However, the permission must still be requested for the foreground service to run reliably.
• "Battery" optimizations also need to be disabled for the foreground service to run reliably, but there's no UI for it. Users need to grant the permission via adb.

On the BasicSync side, there were a couple things to fix:

• Snackbars are not focusable. Compose doesn't provide an easy way to fix this, so we just skip the snackbar button and show the error details dialog immediately.
• The split switch preference's switch element isn't focusable with arrow keys by default (but is focusable with the tab key). This was fixed by manually specifying start/end focus direction targets.