Capture screenshots from the correct window and gate on system memory

[mbarta]

Combines and extends #209 and #216.

Problem

Two independent reasons a destination ends up without a screenshot backdrop during navigation:

• HotwireViewScreenshotHolder always copies from the Activity window. A DialogFragment destination (bottom sheets) draws in its own window, so the capture picks up the screen behind the sheet and shows it inside the sheet as it dismisses (Modal dismiss shows underlying view before closing #150, Allow capturing dialog window content instead of Activities window #216).
• hasEnoughMemoryForScreenshot() compares totalMemory() to maxMemory(), which measures how much the Java heap can still grow, not how much is free. Once the heap has grown to its limit the check is pinned at 0.0 and every capture is refused (Measure used heap, not allocated heap, before capturing a screenshot #209). Bitmap pixels live on the native heap anyway, so the Java heap never reflected the cost being guarded — a 12.6 MB capture on a Pixel emulator moves the native heap by 12.6 MB and the Java heap by 0.

Fix

• HotwireWebFragmentDelegate.screenshotView() resolves the window — the dialog's own window for a DialogFragment, the Activity's otherwise — and passes it to the holder. The capture is skipped when the view is no longer attached, which is the case for the second detach during a dialog cancel.
• The memory check uses ActivityManager.MemoryInfo: skip when lowMemory is set or when the bitmap would push availMem under threshold. Bitmap.createBitmap is wrapped to catch a failed native allocation.
• A skipped capture now logs viewScreenshotSkipped with the values at hand instead of returning silently.

Testing

• HotwireViewScreenshotHolderTest covers the memory gate: headroom, lowMemory, crossing the threshold, and reaching it exactly.
• Verified in the demo app on an emulator: bottom sheet dismiss keeps its own content while sliding out (previously showed the numbers list behind it); full-screen modal push and pop still capture from the Activity window; swipe-down dismiss produces no capture and no skip log.