Color registration buttons correctly <API21

Fixes #6604
Closes #6605
// FREEBIE

res/layout/registration_activity.xml

@@ -104,7 +104,6 @@
                     android:text="@android:string/cancel"
                     android:textColor="@color/white"
                     android:background="@drawable/pill_button"
-                    android:backgroundTint="@color/grey_400"
                     android:textAppearance="?android:attr/textAppearanceMedium"
                     android:layout_width="0dp"
                     android:layout_weight="1"
@@ -119,7 +118,6 @@
                     android:text="@string/registration_activity__register"
                     android:textColor="@color/white"
                     android:background="@drawable/pill_button"
-                    android:backgroundTint="@color/signal_primary"
                     android:textAppearance="?android:attr/textAppearanceMedium"
                     android:layout_width="0dp"
                     android:layout_weight="1"

src/org/thoughtcrime/securesms/RegistrationActivity.java

@@ -3,8 +3,9 @@ package org.thoughtcrime.securesms;
 import android.content.Context;
 import android.content.DialogInterface;
 import android.content.Intent;
+import android.graphics.PorterDuff;
 import android.os.Bundle;
-import android.support.v4.graphics.drawable.DrawableCompat;
+import android.support.v4.content.ContextCompat;
 import android.support.v7.app.AlertDialog;
 import android.text.Editable;
 import android.text.TextUtils;
@@ -95,8 +96,10 @@ public class RegistrationActivity extends BaseActionBarActivity {
     this.informationToggle     =            findViewById(R.id.information_link_container);
     this.informationToggleText = (TextView) findViewById(R.id.information_label);
 
-    DrawableCompat.setTint(this.createButton.getBackground(), getResources().getColor(R.color.signal_primary));
-    DrawableCompat.setTint(this.skipButton.getBackground(), getResources().getColor(R.color.grey_400));
+    this.createButton.getBackground().setColorFilter(ContextCompat.getColor(this, R.color.signal_primary),
+                                                     PorterDuff.Mode.MULTIPLY);
+    this.skipButton.getBackground().setColorFilter(ContextCompat.getColor(this, R.color.grey_400),
+                                                   PorterDuff.Mode.MULTIPLY);
 
     this.countryCode.addTextChangedListener(new CountryCodeChangedListener());
     this.number.addTextChangedListener(new NumberChangedListener());