Feature request: expose navigation bar visibility state / recompute ViewConfig.viewSize on navigation toast

# Problem

With key icons disabled, when a rerouting toast or navigation bar appears mid-ride, the data grid cells physically shrink to make room. However, `startView` is not re-called with updated `ViewConfig` dimensions,  extensions continue rendering with the stale `viewSize` from the original call. Native fields reflow automatically (same-process ConstraintLayout), but extension fields render via cross-process RemoteViews and have no way to detect the container resize. With key icons enabled, the bottom margin is already present at startup, so no resize occurs and the ayout stays correct.

# Impact

Extensions that derive font sizes or vertical positioning from `ViewConfig.viewSize` end up slightly misaligned when the navigation bar appears (~5px shift on Karoo 3). This is noticeable when extension fields sit next to native fields on the same data page.

# Suggested improvements

1. Re-call `startView` with updated `ViewConfig.viewSize` when the cell dimensions change — this would let extensions re-render correctly, matching native field behavior.
2. Alternatively, expose the navigation bar visibility state (showing/hidden) via the SDK event stream, so extensions can adjust their layout independently.

# Environment
- Karoo 3, firmware 1.628
- karoo-ext SDK 1.1.8
- Also observed on Karoo 2