fix: leave the terminal on Back

[oonid]

Back never left the terminal: not the navigation button, not the gesture, not a Back key event. Leaving needed Home or the recents screen.

TerminalActivity's OnBackPressedCallback never ran because the activity never saw a complete Back press:

• with the keyboard up, the IME takes the key-down to hide itself, so only the key-up reaches the activity;
• with the keyboard hidden, Compose takes the key-down to move focus from the terminal library's input view to the Compose root (logged in dispatchKeyEvent: code=4 action=0 handled=true focus=AndroidComposeView).

TerminalActivity.dispatchKeyEvent now handles Back before the view tree: a key-down followed by a key-up runs the back callback (cleanup and finish()), and a lone key-up (the press the IME used to hide itself) is ignored. The first Back still hides the keyboard; the next one leaves. With a text selection active (long press), Back clears it through termlib's public SelectionController (onSelectionControllerAvailable) and stays, as main already did when the key reached termlib; the next Back leaves. docs/android-apk-architecture.md describes it. vendor/termlib is unchanged.

Verification (Android 16, Samsung)

On main, three Back presses (adb key event) left the terminal open, with a live session and after exit. With this change:

• Back with the keyboard up: the first press hides it (mInputShown=false), the second returns to the distro list.
• The navigation bar's Back button: same.
• After exit in the shell: two presses return to the list.
• With a selection: Back clears it (handles and Copy bubble gone) and keeps the session; the next Back leaves.
• The session's proot child is gone after leaving (one child before, none after).
• Integrated with main (fix: map the app's cache group like its real group #10): pr-cli test alpine and pr-cli test ubuntu 55/55, pit all from an in-app terminal 55/55.

Session lifetime (documented here, built in a follow-up)

Back now ends the session, through the existing callback (cleanup()). Before this change the key never reached the callback, so Back left the terminal open and the session alive; an earlier lifecycle check that "survived Back" was measuring that. Because pr-cli starts proot with --kill-on-exit, nothing started inside the session outlives it (nohup, &, setsid do not help).

Action	Session
Back (after the keyboard is hidden)	Ends
exit in the shell	Ends
Home, or switching apps	Keeps running until Android kills the app's process
Swiping the app away	Ends

So a long-running program such as the Hermes gateway needs Home, not Back, and still dies when Android reclaims the process. Options for keeping a session after leaving its screen, recorded in android-apk-architecture.md ("Session lifetime and Back"):

• Back acts like Home (moveTaskToBack): trivial, but lands on the launcher, not the distro list.
• Sessions owned outside the activity: an app-wide session manager owns the PTY, the emulator and the reader thread (which must keep draining output with no screen open); reopening a distro reattaches, the list shows it running, ending becomes an explicit action.
• User choice on top: a setting, a prompt on Back, or a per-session detach. The choice is small; keeping the session is the work.

This PR keeps the documented design (Back ends the session). Background sessions come in a follow-up together with the foreground service, which needs the same session manager and is what stops Android from killing the process.

The app module has no instrumented-test setup, so there is no automated test for this.