Using more StateTriggers

[jsuarezruiz]

After recently added StateTriggers to simplify the management of visual states, in this PR we include more StateTriggers in XamarinTV.

<VisualStateManager.VisualStateGroups>
    <VisualStateGroup Name="PlaybackStates">
        <VisualState Name="Paused">
            <VisualState.StateTriggers>
                <CompareStateTrigger Property="{Binding IsPlaying}" Value="False"/>
            </VisualState.StateTriggers>
            <VisualState.Setters>
                <Setter Property="Source" Value="{StaticResource PlayIcon}"/>
            </VisualState.Setters>
        </VisualState>
        <VisualState Name="Playing">
            <VisualState.StateTriggers>
                <CompareStateTrigger Property="{Binding IsPlaying}" Value="True"/>
            </VisualState.StateTriggers>
            <VisualState.Setters>
                <Setter Property="Source" Value="{StaticResource PauseIcon}"/>
            </VisualState.Setters>
        </VisualState>
    </VisualStateGroup>
</VisualStateManager.VisualStateGroups>