convert top level classes to kotlin (#6368)

* Converted welcome activity / pager to kotlin

* Removed unused interface

* Convert ViewPagerAdapter to kotlin and enforce that all tabs must have a title that comes from strings.xml

* Convert OkHttpConnectionFactory and remove an exception class nobody was using

* Convert MapController to kotlin along with fixing nullability in a few places

app/src/main/java/fr/free/nrw/commons/WelcomePagerAdapter.kt

@@ -0,0 +1,70 @@
+package fr.free.nrw.commons
+
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import android.widget.TextView
+import androidx.core.net.toUri
+import androidx.viewpager.widget.PagerAdapter
+import fr.free.nrw.commons.utils.UnderlineUtils.setUnderlinedText
+import fr.free.nrw.commons.utils.handleWebUrl
+
+class WelcomePagerAdapter : PagerAdapter() {
+    /**
+     * Gets total number of layouts
+     * @return Number of layouts
+     */
+    override fun getCount(): Int = PAGE_LAYOUTS.size
+
+    /**
+     * Compares given view with provided object
+     * @param view Adapter view
+     * @param obj Adapter object
+     * @return Equality between view and object
+     */
+    override fun isViewFromObject(view: View, obj: Any): Boolean = (view === obj)
+
+    /**
+     * Provides a way to remove an item from container
+     * @param container Adapter view group container
+     * @param position Index of item
+     * @param obj Adapter object
+     */
+    override fun destroyItem(container: ViewGroup, position: Int, obj: Any) =
+        container.removeView(obj as View)
+
+    override fun instantiateItem(container: ViewGroup, position: Int): Any {
+        val inflater = LayoutInflater.from(container.context)
+        val layout = inflater.inflate(PAGE_LAYOUTS[position], container, false) as ViewGroup
+
+        // If final page
+        if (position == PAGE_LAYOUTS.size - 1) {
+            // Add link to more information
+            val moreInfo = layout.findViewById<TextView>(R.id.welcomeInfo)
+            setUnderlinedText(moreInfo, R.string.welcome_help_button_text)
+            moreInfo.setOnClickListener {
+                handleWebUrl(
+                    container.context,
+                    "https://commons.wikimedia.org/wiki/Help:Contents".toUri()
+                )
+            }
+
+            // Handle click of finishTutorialButton ("YES!" button) inside layout
+            layout.findViewById<View>(R.id.finishTutorialButton)
+                .setOnClickListener { view: View? -> (container.context as WelcomeActivity).finishTutorial() }
+        }
+
+        container.addView(layout)
+        return layout
+    }
+
+    companion object {
+        private val PAGE_LAYOUTS = intArrayOf(
+            R.layout.welcome_wikipedia,
+            R.layout.welcome_do_upload,
+            R.layout.welcome_dont_upload,
+            R.layout.welcome_image_example,
+            R.layout.welcome_final
+        )
+    }
+}