Remove usage of alpha color selectors (fixes #2957)

Doesn't work properly in API < 23

app/src/main/java/eu/kanade/tachiyomi/ui/source/browse/SourceListHolder.kt

@@ -1,7 +1,6 @@
 package eu.kanade.tachiyomi.ui.source.browse
 
 import android.view.View
-import androidx.core.content.ContextCompat
 import com.bumptech.glide.load.engine.DiskCacheStrategy
 import eu.davidea.flexibleadapter.FlexibleAdapter
 import eu.kanade.tachiyomi.R
@@ -23,7 +22,7 @@ import kotlinx.android.synthetic.main.source_list_item.title
 class SourceListHolder(private val view: View, adapter: FlexibleAdapter<*>) :
     SourceHolder(view, adapter) {
 
-    private val favoriteColor = ContextCompat.getColor(view.context, R.color.material_on_surface_disabled)
+    private val favoriteColor = view.context.getResourceColor(R.attr.colorOnSurface, 0.38f)
     private val unfavoriteColor = view.context.getResourceColor(R.attr.colorOnSurface)
 
     /**

app/src/main/java/eu/kanade/tachiyomi/ui/source/filter/TriStateItem.kt

@@ -2,7 +2,6 @@ package eu.kanade.tachiyomi.ui.source.filter
 
 import android.view.View
 import android.widget.CheckedTextView
-import androidx.core.content.ContextCompat
 import androidx.recyclerview.widget.RecyclerView
 import androidx.vectordrawable.graphics.drawable.VectorDrawableCompat
 import com.google.android.material.R
@@ -46,7 +45,7 @@ open class TriStateItem(val filter: Filter.TriState) : AbstractFlexibleItem<TriS
             val color = if (filter.state == Filter.TriState.STATE_INCLUDE) {
                 view.context.getResourceColor(R.attr.colorAccent)
             } else {
-                ContextCompat.getColor(view.context, R.color.material_on_background_disabled)
+                view.context.getResourceColor(R.attr.colorOnBackground, 0.38f)
             }
 
             setTint(color)