Android: add Back button listener #1564
Description
Currently tauri and frameworks that rely on wry/tao such as dioxus don't seem to support receiving back button presses. This could be useful to allow back button navigation within the exposed web-view.
Proposed solution
Receive back button presses in the WryActivity (using androidx's OnBackPressedDispatcher .addCallback) and call a function in the ndk_glue (in tao). A separate crate, that client code can depend on, should expose an API that notifies subscribers of back button presses.
Alternatives
The back button presses could also be directly forwarded to the webview. If the webview has nowhere to go back to, it should close the app.
This issue is similar to #1563, except that it cannot be handled in another activity.