Extend assist context to foreground services

Add Service.onProvideAssistData(Bundle) which will be
called on foreground Services that have the new attr
in their manifest of provideAssistData = true;

Rename private reference to e.g. "getTopActivityExtras"
as "getAssistContextExtras" - do not rename the relevant
permission, since it is already public.

In ActivityManagerService, request extras both from the
top activity and from any foreground services with the
above attribute. Extend PendingActivityExtras as
PendingAssistExtras with a list of Services from which
extras are expected.

Reduce the timeout to or reporting extras from 4 sec to
just 500 ms.

Bug: 9526331
Change-Id: Ia03b96e8189033a68ae9c514c8cea0199a19bce8