fix: restore in-app back navigation under predictive back

[fortuna]

What

Fixes in-app back navigation, which is broken on master right now, and resolves the
GestureBackNavigation lint error.

Important

This is a follow-up to #623 (already merged). That PR bumped targetSdk to 36, which is
what introduced this regression. This PR is based directly on master and is
independent of the AGP stack (#625/#626/#627), so it can merge on its own.

The bug

MainActivity overrode onBackPressed() so that back returns to the home view from the
settings and info screens:

  @Override
  public void onBackPressed() {
    if (isShowingHomeView()) {
      super.onBackPressed();
      return;
    }
    chooseView(R.id.frame_main);
  }

Apps targeting SDK 36 can no longer opt out of predictive back, and the platform
routes back gestures through OnBackPressedDispatcher instead of calling
onBackPressed(). So pressing back from Settings exits the app rather than returning
to the home view.

This is specific to targetSdk 36 — it is not a pre-existing bug. Running lintDebug
across targetSdk values on an otherwise identical tree:

targetSdk	total lint errors	GestureBackNavigation
33	3	not reported
34	4	not reported
35	4	not reported
36	5	reported

targetSdk 35 (Android 15) turned on predictive back animations but kept the legacy
onBackPressed() path working. targetSdk 36 (Android 16) removes that compatibility path.

The fix

Register an OnBackPressedCallback on the dispatcher instead, enabled only while a
non-home view is showing:

  private final OnBackPressedCallback backToHomeCallback = new OnBackPressedCallback(false) {
    @Override
    public void handleOnBackPressed() {
      chooseView(R.id.frame_main);
    }
  };

chooseView() is the single funnel for every view change, so it is the natural place to
keep the enabled state in sync. It sits right next to the existing up-arrow logic, which
already encodes the same "am I on the home view" condition:

    backToHomeCallback.setEnabled(id != R.id.frame_main);

When disabled, back falls through to the system and leaves the app, preserving the
original behavior. isShowingHomeView() has no remaining callers and is removed.

Why AppCompat moves 1.4.0 -> 1.7.1

Bridging the platform's OnBackInvokedCallback into OnBackPressedDispatcher needs
androidx.activity 1.6+. The pinned appcompat 1.4.0 resolved activity:1.2.4, which
predates predictive back entirely. appcompat 1.7.1 pulls activity:1.8.0.

Note

appcompat 1.8.0 is newer, but requires raising minSdk to 23:

uses-sdk:minSdkVersion 21 cannot be smaller than version 23 declared in library
[androidx.appcompat:appcompat-resources:1.8.0]

1.7.1 is used deliberately so that minSdk stays at 21 and Android 5.x users are not
dropped. For a censorship-circumvention tool, old devices are worth keeping.

gradle/verification-metadata.xml is updated with the new transitive dependencies
(activity 1.8.0, lifecycle 2.6.x, core 1.13.0, emoji2, savedstate, profileinstaller, ...).

Verification

• ./gradlew clean assembleDebug test passes.
• GestureBackNavigation is gone. The remaining lint errors are the 3 pre-existing ones
  (StartActivityAndCollapseDeprecated, RestrictedApi, QueryAllPackagesPermission).

Exercised on a Pixel 8 emulator running API 36 (the configuration where the bug
reproduces), driving the UI over adb + uiautomator:

Check	Result
Drawer -> Settings shows the settings view	pass
Back from Settings stays in the app	pass
Back from Settings returns to the home view	pass
Tapping a stat box shows the info view	pass
Back from the info page stays in the app	pass
Back from the info page returns to the home view	pass
Back from the home view leaves the app	pass
No crashes in logcat	pass

Both chooseView() entry points are covered: the settings view and the info view. Back
from the home view still exits, so the pre-existing behavior is preserved.

[jyyi1]

LGTM, thanks!