remove ripple effect and make buttons transparent

- they will inherit the backgrounds' color

app/src/main/res/drawable-v21/button.xml

@@ -1,5 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<ripple xmlns:android="http://schemas.android.com/apk/res/android"
-        android:color="@color/dark_grey">
-    <item android:drawable="@drawable/button_normal"/>
-</ripple>

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

@@ -3,7 +3,4 @@
     <item
         android:drawable="@drawable/button_pressed"
         android:state_pressed="true"/>
-    <item
-        android:drawable="@drawable/button_normal"
-        android:state_pressed="false"/>
 </selector>

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

@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<shape xmlns:android="http://schemas.android.com/apk/res/android"
-       android:shape="rectangle">
-    <solid
-        android:color="@color/dark_grey"/>
-</shape>

app/src/main/res/layout/activity_main.xml

@@ -9,6 +9,7 @@
     <LinearLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent"
+        android:background="@color/dark_grey"
         android:orientation="vertical">
 
         <TextView
@@ -16,7 +17,6 @@
             android:layout_width="match_parent"
             android:layout_height="match_parent"
             android:layout_weight="1.8"
-            android:background="@color/dark_grey"
             android:gravity="center_vertical|right"
             android:lines="1"
             android:paddingLeft="@dimen/activity_margin"

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

@@ -4,5 +4,5 @@
     <color name="colorPrimaryDark">#303F9F</color>
     <color name="colorAccent">#FF4081</color>
     <color name="dark_grey">#88000000</color>
-    <color name="dark_grey_pressed">#99000000</color>
+    <color name="dark_grey_pressed">#22000000</color>
 </resources>