Clean up ClassPathPackageInfoSource

Part of the work of removing JUnit and dependent android.test classes
from the Android API involves providing a static library that developers
can include in their test applications to ease migration. That library
will be built directly from the source (as opposed to android.jar which
is built from stubs) and so developers will be able to see classes and
methods that are not present in the stubs. This change is one of a
number of similar changes that cleanup the existing non-API code in
order to minimize the additional methods and classes exposed externally.
The basic approach is to remove unused classes and methods, use least
visible access modifier possible and generally minimize the amount of
publicly visible code.

PackageInfoSources only provided a static field and accessor method so
they were moved into ClassPathPackageInfoSource and PackageInfoSources
was removed.

ClassPathPackageInfo was only used in ClassPathPackageInfoSource and in
TestGrouping. In the latter it was simply used as an intermediate value
between ClassPathPackageInfoSource.getPackage(String packageName) and
ClassPathPackageInfo.getTopLevelClassesRecursive(String packageName).
Moving that method into ClassPathPackageInfoSource allowed the
ClassPathPackageInfo to become an inner class of
ClassPathPackageInfoSource. As it is an inner class it no longer needed
an explicit reference to the containing ClassPathPackageInfoSource.

Bug: 30188076
Test: make checkbuild and ran FrameworkTestRunnerTests
Change-Id: Idb0b6a585030805b9cff8562abb93b7e5920c53a