Use vector drawables instead of PNGs for material icons

For all manually-created images PNG have been kept.
- rename all icon attrs to have a `ic_` prefix
- always use `_24dp` icons, because there is no real difference, since they are vector drawables
- always use the original name found on material.io for icon drawables, as to not create confusion and possibly duplicates. Icon names can still be different from real drawable names, though I have made some of them compliant to this or maybe more meaningul.
- remove duplicate `getIconByAttr()` in ThemeHelper (use `resolveResourceIdFromAttr()`
- use standard icons for `expand_more` and `expand_less` instead of triangles
- use `play_button_outline` instead of custom PNG as play button in VideoDetailFragment (questionable, as there is no shadow anymore)

app/src/main/java/org/schabi/newpipe/settings/tabs/Tab.java

@@ -231,7 +231,7 @@ public abstract class Tab {
         @DrawableRes
         @Override
         public int getTabIconRes(final Context context) {
-            return ThemeHelper.resolveResourceIdFromAttr(context, R.attr.rss);
+            return ThemeHelper.resolveResourceIdFromAttr(context, R.attr.ic_rss);
         }
 
         @Override
@@ -281,7 +281,7 @@ public abstract class Tab {
         @DrawableRes
         @Override
         public int getTabIconRes(final Context context) {
-            return ThemeHelper.resolveResourceIdFromAttr(context, R.attr.history);
+            return ThemeHelper.resolveResourceIdFromAttr(context, R.attr.ic_history);
         }
 
         @Override