Clean up password unlock layout

Remove unnecessary and redundant nested layouts, fix the alignment of
the unlock button to the right edge of the screen, use cleaner margins
and replace the deprecated fill_parent sizes with match_parent.

res/layout/prompt_passphrase_activity.xml

@@ -1,57 +1,43 @@
 <?xml version="1.0" encoding="utf-8"?>
 
 <ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
-            android:layout_width="fill_parent"
-            android:layout_height="fill_parent"
+            android:layout_width="match_parent"
+            android:layout_height="match_parent"
             android:fillViewport="true"
             android:background="@drawable/background_pattern_repeat">
 
-    <FrameLayout
-        android:layout_width="fill_parent"
-        android:layout_height="fill_parent"
-        android:gravity="center" >
-        
     <LinearLayout android:id="@+id/prompt_layout"
-                      android:paddingRight="16dip"
-                      android:paddingLeft="16dip"
-                      android:paddingTop="10dip"
-                      android:layout_width="fill_parent"
+                  android:padding="16dp"
+                  android:layout_width="match_parent"
                   android:layout_height="wrap_content"
-                      android:layout_gravity="center"
+                  android:gravity="center_horizontal|center_vertical"
                   android:orientation="vertical">
 
         <ImageView android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
-                       android:layout_marginBottom="30dip"
+                   android:layout_marginBottom="24dp"
                    android:src="@drawable/padlock_prompt"/>
 
         <TextView style="@style/Registration.Label"
-                      android:layout_width="fill_parent"
+                  android:layout_width="match_parent"
                   android:textAllCaps="true"
                   android:text="@string/prompt_passphrase_activity__textsecure_passphrase" />
 
         <EditText android:id="@+id/passphrase_edit"
-                      android:layout_width="fill_parent"
+                  android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:inputType="textPassword"
-                      android:layout_marginBottom="10dip"
+                  android:layout_marginBottom="8dp"
                   android:singleLine="true"/>
 
-            <LinearLayout android:orientation="horizontal"
-                          android:gravity="right"
-                          android:layout_width="fill_parent"
-                          android:layout_height="wrap_content"
-                          android:layout_marginTop="20dip"
-                          android:layout_marginRight="20dip">
-                
         <Button android:id="@+id/ok_button"
                 android:text="@string/prompt_passphrase_activity__unlock"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:layout_gravity="right"
                 android:textAppearance="?android:attr/textAppearanceMedium"/>
+
     </LinearLayout>
-        </LinearLayout>        
-    </FrameLayout>
+
 </ScrollView>