Remove usage of alpha color selectors (fixes #2957)

Doesn't work properly in API < 23

app/src/main/java/eu/kanade/tachiyomi/util/system/ContextExtensions.kt

@@ -9,11 +9,13 @@ import android.content.Intent
 import android.content.IntentFilter
 import android.content.pm.PackageManager
 import android.content.res.Resources
+import android.graphics.Color
 import android.net.ConnectivityManager
 import android.net.Uri
 import android.os.PowerManager
 import android.widget.Toast
 import androidx.annotation.AttrRes
+import androidx.annotation.ColorInt
 import androidx.annotation.StringRes
 import androidx.browser.customtabs.CustomTabsIntent
 import androidx.core.app.NotificationCompat
@@ -22,6 +24,7 @@ import androidx.localbroadcastmanager.content.LocalBroadcastManager
 import com.nononsenseapps.filepicker.FilePickerActivity
 import eu.kanade.tachiyomi.R
 import eu.kanade.tachiyomi.widget.CustomLayoutPickerActivity
+import kotlin.math.roundToInt
 
 /**
  * Display a toast in this context.
@@ -96,12 +99,22 @@ fun Context.hasPermission(permission: String) = ContextCompat.checkSelfPermissio
  * Returns the color for the given attribute.
  *
  * @param resource the attribute.
+ * @param alphaFactor the alpha number [0,1].
  */
-fun Context.getResourceColor(@AttrRes resource: Int): Int {
+@ColorInt fun Context.getResourceColor(@AttrRes resource: Int, alphaFactor: Float = 1f): Int {
     val typedArray = obtainStyledAttributes(intArrayOf(resource))
-    val attrValue = typedArray.getColor(0, 0)
+    val color = typedArray.getColor(0, 0)
     typedArray.recycle()
-    return attrValue
+
+    if (alphaFactor < 1f) {
+        val alpha = (Color.alpha(color) * alphaFactor).roundToInt()
+        val red = Color.red(color)
+        val green = Color.green(color)
+        val blue = Color.blue(color)
+        return Color.argb(alpha, red, green, blue)
+    }
+
+    return color
 }
 
 /**