ExtView: Add samples for using KeyguardExternalViews

This adds two samples, samples/externalviews/keyguardviewprovider
which provides an example of creating a third party keyguard component,
and samples/externalviews/setleygiardviewprovider which allows setting
and clearing of the third party keyguard component.

Change-Id: Ia9c564d602f1b6d44953eb05f2ffcf3f44663ece