Themes: Support theming navbar separately from SystemUI

This allows using a different theme for the navbar than the theme
applied to the rest of SystemUI.  This will not affect theming until
we have the front end in place that allows users to change this.

Credit to @BigBrother1984 for the original cherry pick

Change-Id: I753623696cdc017fc66dc5e85079fe80e5f626f9
Add getThemedResourcesForApplicationAsUser()

Credit to @BigBrother1984 for the original cherry pick

Change-Id: I90b7cb441e4056f59b7260ffa55e31eba644e25a
Themes: Allow specifying a theme when creating a Context

A Context normally loads the resources associated with the given
package that this context is for.  This patch allows us to specify
a theme we want attached to a given context's resources.

Credit to @BigBrother1984 for the original cherry pick

Change-Id: Idfb53aba307f58be6e2ed6d404b8a76c31199fc8