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/util/KioskTranslator.java

@@ -61,7 +61,7 @@ public final class KioskTranslator {
             case "Recently added":
                 return ThemeHelper.resolveResourceIdFromAttr(c, R.attr.ic_kiosk_recent);
             case "Most liked":
-                return ThemeHelper.resolveResourceIdFromAttr(c, R.attr.thumbs_up);
+                return ThemeHelper.resolveResourceIdFromAttr(c, R.attr.ic_thumb_up);
             default:
                 return 0;
         }

app/src/main/java/org/schabi/newpipe/util/ThemeHelper.java

@@ -231,16 +231,4 @@ public final class ThemeHelper {
         return PreferenceManager.getDefaultSharedPreferences(context)
                 .getString(themeKey, defaultTheme);
     }
-
-    /**
-     * This will get the R.drawable.* resource to which attr is currently pointing to.
-     *
-     * @param attr    a R.attribute.* resource value
-     * @param context the context to use
-     * @return a R.drawable.* resource value
-     */
-    public static int getIconByAttr(final int attr, final Context context) {
-        return context.obtainStyledAttributes(new int[]{attr})
-                .getResourceId(0, -1);
-    }
 }