Improve the drawing screen: going back/discarding

[ZornHadNoChoice]

• Swipe gesture does not trigger the discard dialog.
• The back button triggers the "Go back again to exit" snackbar.

Screenrecorder-2026-05-03-04-26-09-183.mp4

AnkiDroid Version = 2.24.0 (ebcf8e0e34921628b9b8a496c66ffd4adbb3705f)  
Backend Version = 0.1.64-anki25.09.2 (25.09.2 3890e12c9e48c028c3f12aa58cb64bd9f8895e30)  
Android Version = 13 (SDK 33)  
ProductFlavor = full  
Device Info = Xiaomi | POCO | mondrian | mondrian_global | 23013PC75G | qcom  
WebView Info = [com.google.android.webview | 772711103]: Mozilla/5.0 (Linux; Android 13; 23013PC75G Build/TKQ1.220905.001; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/147.0.7727.111 Mobile Safari/537.36  
ACRA UUID = 4afc3b3f-e93f-4a66-bc71-2b9476ef9eb0  
FSRS = 5.1.0 (Enabled: true)  
Crash Reports Enabled = true

[ZornHadNoChoice]

This is kind of my fault since I suggested that the drawing screen should also have the "Go back again to exit" in #20922.

Now it seems more reasonable to just show the discard dialog instead.