Android overlay a view ontop of everything?

Simply use RelativeLayout or FrameLayout. The last child view will overlay everything else.

Android supports a pattern which Cocoa Touch SDK doesn't: Layout management.
Layout for iPhone means to position everything absolute (besides some strech factors). Layout in android means that children will be placed in relation to eachother.

Example (second EditText will completely cover the first one):

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:id="@+id/root_view">

    <EditText
        android:layout_width="fill_parent"
        android:id="@+id/editText1"
        android:layout_height="fill_parent">
    </EditText>

    <EditText
        android:layout_width="fill_parent"
        android:id="@+id/editText2"
        android:layout_height="fill_parent">
        <requestFocus></requestFocus>
    </EditText>

</FrameLayout>

FrameLayout is some kind of view stack. Made for special cases.

RelativeLayout is pretty powerful. You can define rules like View A has to align parent layout bottom, View B has to align A bottom to top, etc

Update based on comment

Usually you set the content with setContentView(R.layout.your_layout) in onCreate (it will inflate the layout for you). You can do that manually and call setContentView(inflatedView), there's no difference.

The view itself might be a single view (like TextView) or a complex layout hierarchy (nested layouts, since all layouts are views themselves).

After calling setContentView your activity knows what its content looks like and you can use (FrameLayout) findViewById(R.id.root_view) to retrieve any view int this hierarchy (General pattern (ClassOfTheViewWithThisId) findViewById(R.id.declared_id_of_view)).

You can use bringToFront:

    View view=findViewById(R.id.btnStartGame);
    view.bringToFront();

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/root_view"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >

<LinearLayout
    android:id = "@+id/Everything"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
    <!-- other actual layout stuff here EVERYTHING HERE -->

   </LinearLayout>

<LinearLayout
    android:id="@+id/overlay"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="right" >
</LinearLayout>

Now any view you add under LinearLayout with android:id = "@+id/overlay" will appear as overlay with gravity = right on Linear Layout with android:id="@+id/Everything"