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/res/xml/main_settings.xml

@@ -8,31 +8,31 @@
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.VideoAudioSettingsFragment"
-        android:icon="?attr/audio"
+        android:icon="?attr/ic_headset"
         android:title="@string/settings_category_video_audio_title"/>
 
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.DownloadSettingsFragment"
-        android:icon="?attr/download"
+        android:icon="?attr/ic_file_download"
         android:title="@string/settings_category_downloads_title"/>
 
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.AppearanceSettingsFragment"
-        android:icon="?attr/palette"
+        android:icon="?attr/ic_palette"
         android:title="@string/settings_category_appearance_title"/>
 
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.HistorySettingsFragment"
-        android:icon="?attr/history"
+        android:icon="?attr/ic_history"
         android:title="@string/settings_category_history_title"/>
 
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.ContentSettingsFragment"
-        android:icon="?attr/language"
+        android:icon="?attr/ic_language"
         android:title="@string/content"/>
 
     <PreferenceScreen
@@ -45,7 +45,7 @@
     <PreferenceScreen
         app:iconSpaceReserved="false"
         android:fragment="org.schabi.newpipe.settings.DebugSettingsFragment"
-        android:icon="?attr/bug"
+        android:icon="?attr/ic_bug_report"
         android:title="@string/settings_category_debug_title"
         android:key="@string/debug_pref_screen_key"/>
 </PreferenceScreen>