Fixing gradient issue on older android versions (#1069)

paprently  older versions of android can't use gradients with a mix of hex colors and dynamic theme attributes

app/src/main/res/drawable/fade_gradient.xml

@@ -5,7 +5,7 @@
 
     <gradient
         android:centerY="0.75"
-        android:centerColor="#00000000"
+        android:centerColor="?transparent"
         android:endColor="?colorPrimary"
         android:angle="270" />
 

app/src/main/res/values/attrs.xml

@@ -23,6 +23,7 @@
     <attr name="textColorAlert" format="reference|color"/>
     <attr name="elementBorderColor" format="reference|color"/>
     <attr name="conversation_background" format="reference|color"/>
+    <attr name="transparent" format="reference|color"/>
 
     <attr name="emoji_text_color" format="color" />
 

app/src/main/res/values/themes.xml

@@ -43,6 +43,7 @@
        <item name="onInvertedBackgroundPrimary">?colorPrimary</item>
        <item name="prominentButtonColor">?colorAccent</item>
        <item name="colorError">?danger</item>
+       <item name="transparent">@color/transparent</item>
 
        <item name="android:actionMenuTextAppearance">@style/MenuTextAppearance</item>
    </style>