Added client requests for ImpressionAttestation

Since the ImpressionAttestation requests will be from outside the
system, exposed some methods in IWindowManager and IWindowSession to
allow apps to generate the impression token and allow the ads services
to validate the token.

Most of the methods are passthroughs to ExtServices since client apps
cannot directly call into ExtServices

For the generateImpressionToken method, WindowManager will look up the
information about the window to find it's location in the display. It
will then update the bounds passed in so they are in screen space
instead of window space. WindowManager will take the screenshot, using
the uid from the window session, and send the screenshot to ExtServices.
That's where the hash and token will be generated.

Test: Builds
Bug: 155825630
Change-Id: Iacc6c398794d9a302e4d609a16fa99a810369aff