[Fix]in sample application text's are not readable in dark theme (#1942)

sample/src/main/res/values-v21/styles_parent.xml

@@ -4,11 +4,13 @@
   <style name="ThemeParent" parent="Theme.MaterialComponents.Light.DarkActionBar">
     <item name="android:navigationBarColor">?colorPrimaryDark</item>
     <item name="sample_button_color">#80000000</item>
+    <item name="sample_button_text_color">#e6e6e6</item>
   </style>
 
   <style name="ThemeParent.Dark" parent="Theme.MaterialComponents">
     <item name="android:navigationBarColor">?colorPrimaryDark</item>
     <item name="sample_button_color">#80000000</item>
+    <item name="sample_button_text_color">#e6e6e6</item>
   </style>
 
 </resources>

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

 <?xml version="1.0" encoding="utf-8"?>
 <resources>
   <attr format="color" name="sample_button_color"/>
+  <attr format="color" name="sample_button_text_color"/>
 </resources>

sample/src/main/res/values/styles.xml

@@ -49,6 +49,7 @@
     <item name="android:layout_width">match_parent</item>
     <item name="android:layout_height">@dimen/sample_button_height</item>
     <item name="android:layout_marginTop">@dimen/sample_button_spacing</item>
+    <item name="android:textColor">?sample_button_text_color</item>
     <item name="backgroundTint">?sample_button_color</item>
   </style>
 

sample/src/main/res/values/styles_parent.xml

@@ -3,10 +3,12 @@
 
   <style name="ThemeParent" parent="Theme.MaterialComponents.Light.DarkActionBar">
     <item name="sample_button_color">#80000000</item>
+    <item name="sample_button_text_color">#e6e6e6</item>
   </style>
 
   <style name="ThemeParent.Dark" parent="Theme.MaterialComponents">
     <item name="sample_button_color">#80000000</item>
+    <item name="sample_button_text_color">#e6e6e6</item>
   </style>
 
 </resources>