fixed
Java-friendly factory for a Custom inset reserving a fixed top strip of topPx physical pixels at density (bottom/start/end zero). The SDK host uses this to mirror the View-side toolbar slot (Dp is a Kotlin value class, so Custom can't be constructed from Java directly).
Reserves the side system bars (Custom.includesSideSystemBars = true): this describes the SDK's own edge-to-edge mounting, where only the top strip varies and the landscape navigation bar always has to be cleared.