feat: Hide 'Hard' and 'Easy' buttons setting

It is a frequently requested feature, helps a big number of users that
only use two buttons, and it is quite simple to implement

AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt

@@ -215,6 +215,11 @@ class ReviewerFragment :
                 answerButtonsLayout.isVisible = false
             }
         }
+
+        if (sharedPrefs().getBoolean(getString(R.string.hide_hard_and_easy_key), false)) {
+            hardButton.isVisible = false
+            easyButton.isVisible = false
+        }
     }
 
     private fun setupCounts(view: View) {

AnkiDroid/src/main/res/values/10-preferences.xml

@@ -444,4 +444,5 @@ this formatter is used if the bind only applies to both the question and the ans
         >All</string>
     <string name="ignore_display_cutout" maxLength="41">Ignore display cutout</string>
     <string name="hide_answer_buttons" maxLength="41">Hide answer buttons</string>
+    <string name="hide_hard_and_easy" maxLength="41">Hide ‘Hard’ and ‘Easy’ buttons</string>
 </resources>

AnkiDroid/src/main/res/values/preferences.xml

@@ -201,5 +201,6 @@
     <string name="hide_system_bars_key">hideSystemBars</string>
     <string name="ignore_display_cutout_key">ignoreDisplayCutout</string>
     <string name="hide_answer_buttons_key">hideAnswerButtons</string>
+    <string name="hide_hard_and_easy_key">hideHardAndEasy</string>
 
 </resources>

AnkiDroid/src/main/res/xml/preferences_reviewer.xml

@@ -21,4 +21,10 @@
         android:key="@string/hide_answer_buttons_key"
         android:title="@string/hide_answer_buttons"
         />
+
+    <SwitchPreferenceCompat
+        android:defaultValue="false"
+        android:key="@string/hide_hard_and_easy_key"
+        android:title="@string/hide_hard_and_easy"
+        />
 </androidx.preference.PreferenceScreen>