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/download/DownloadDialog.java

@@ -399,8 +399,8 @@ public class DownloadDialog extends DialogFragment
         boolean isLight = ThemeHelper.isLightThemeSelected(getActivity());
 
         toolbar.setTitle(R.string.download_dialog_title);
-        toolbar.setNavigationIcon(isLight ? R.drawable.ic_arrow_back_black_24dp
-                : R.drawable.ic_arrow_back_white_24dp);
+        toolbar.setNavigationIcon(
+            ThemeHelper.resolveResourceIdFromAttr(requireContext(), R.attr.ic_arrow_back));
         toolbar.inflateMenu(R.menu.dialog_url);
         toolbar.setNavigationOnClickListener(v -> getDialog().dismiss());
         toolbar.setNavigationContentDescription(R.string.cancel);