Add setting to allow configuration of long press home gesture

This allows configuration of long press home gesture on 2-/3-button navigation configurations. It allows the user to enable and disable the invocation of assistant app using said gesture.

Bug: 191888710
Test: Unit tests and on a physical device.
Change-Id: I18cb290058ac5c2a748d40802b942404f548b868
diff --git a/src/com/android/settings/Settings.java b/src/com/android/settings/Settings.java
index 231a149..4de795c 100644
--- a/src/com/android/settings/Settings.java
+++ b/src/com/android/settings/Settings.java
@@ -243,6 +243,8 @@
     public static class AutomaticStorageManagerSettingsActivity extends SettingsActivity { /* empty */ }
     public static class GamesStorageActivity extends SettingsActivity { /* empty */ }
     public static class GestureNavigationSettingsActivity extends SettingsActivity { /* empty */ }
+    /** Activity to manage 2-/3-button navigation configuration. */
+    public static class ButtonNavigationSettingsActivity extends SettingsActivity { /* empty */ }
     public static class InteractAcrossProfilesSettingsActivity extends SettingsActivity {
         /* empty */
     }