Homepage improvements (closes #29, #64, #146, #171) (#184)

Some improvements to the Homescreen!

* Do not show collections placeholder in home screen on a clean install and subsequently as well. 
* Increase the limit for top sites to 160
* Show 20 top sites on the screen at a time, before pagination occurs.

app/src/main/java/org/mozilla/fenix/home/HomeFragment.kt

@@ -208,7 +208,7 @@ class HomeFragment : Fragment() {
                             )
                         ).getTip()
                     },
-                    showCollectionPlaceholder = components.settings.showCollectionsPlaceholderOnHome
+                    showCollectionPlaceholder = false
                 )
             )
         }

app/src/main/java/org/mozilla/fenix/home/sessioncontrol/viewholders/TopSitePagerViewHolder.kt

@@ -61,6 +61,6 @@ class TopSitePagerViewHolder(
     companion object {
         const val LAYOUT_ID = R.layout.component_top_sites_pager
         const val TOP_SITES_MAX_PAGE_SIZE = 2
-        const val TOP_SITES_PER_PAGE = 8
+        const val TOP_SITES_PER_PAGE = 20
     }
 }

app/src/main/java/org/mozilla/fenix/utils/Settings.kt

@@ -52,7 +52,7 @@ private const val AUTOPLAY_USER_SETTING = "AUTOPLAY_USER_SETTING"
 class Settings(private val appContext: Context) : PreferencesHolder {
 
     companion object {
-        const val topSitesMaxCount = 16
+        const val topSitesMaxCount = 160
         const val FENIX_PREFERENCES = "fenix_preferences"
 
         private const val BLOCKED_INT = 0