[fenix] For https://github.com/mozilla-mobile/fenix/issues/5872 & https://github.com/mozilla-mobile/fenix/issues/6075: Set TabHeader buttons to invisible instead of gone.

At least one button has to be invisible instead of gone to keep layout height.
Tabs overflow button kept gone to avoid empty space on view end in private mode.

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

@@ -7,6 +7,7 @@ package org.mozilla.fenix.home.sessioncontrol.viewholders
 import android.content.Context
 import android.view.View
 import android.widget.PopupWindow
+import androidx.core.view.isInvisible
 import androidx.core.view.isVisible
 import androidx.recyclerview.widget.RecyclerView
 import io.reactivex.Observer
@@ -81,8 +82,8 @@ class TabHeaderViewHolder(
         val headerTextResourceId =
             if (isPrivate) R.string.tabs_header_private_title else R.string.tab_header_label
         view.header_text.text = view.context.getString(headerTextResourceId)
-        view.share_tabs_button.isVisible = isPrivate && hasTabs
-        view.close_tabs_button.isVisible = isPrivate && hasTabs
+        view.share_tabs_button.isInvisible = !isPrivate || !hasTabs
+        view.close_tabs_button.isInvisible = !isPrivate || !hasTabs
         view.tabs_overflow_button.isVisible = !isPrivate && hasTabs
     }