Add an API to query the package name of the services shared lib

The servcies shared lib contains components apps can invoke such
as services to bind to, activities to start, UI choosers, etc.
This lib is built from AOSP code but an OEM may chage its
package name. For example, Google renames the package names for
GMS apps from android.foo.bar to com.google.android.foo.bar.
While we have more than one shared lib that are a part of the
platform (currently shared and services libs) the serivces lib
is the only one clients need to start components in, thus need
to know its package name. This change adds an API to query the
package name of the services shared lib. The API is hidden as
currently the only clients are a part of the system.

Change-Id: Ied48fa4819024522791764b22b3336d4f4b42cc3