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/us/shandian/giga/util/Utility.java

@@ -191,12 +191,12 @@ public class Utility {
     public static int getIconForFileType(FileType type) {
         switch (type) {
             case MUSIC:
-                return R.drawable.music;
+                return R.drawable.ic_headset_white_24dp;
             default:
             case VIDEO:
-                return R.drawable.video;
+                return R.drawable.ic_movie_white_24dp;
             case SUBTITLE:
-                return R.drawable.subtitle;
+                return R.drawable.ic_subtitles_white_24dp;
         }
     }