Add gravity end to the volume dialog root view

The problem is the window behaves differently depending on the
navigation mode enabled. When there is a 3-btn navigation enabled the
window was some minimum width it fills despite being WRAP_CONTENT.

This also fixed the navigation scrim overlay appearing for 3-btn
navigation by disabling it in the theme.

Flag: com.android.systemui.volume_redesign
Fixes: 406620015
Test: atest VolumeDialogScreenshotTest
Test: manula on phone. Force fullscreen volume dialog window and observe
its position

Change-Id: I0761ea04e46312377c504f10b335a59d68df5d92