Refactor the PackageBackwardCompatibilityTest

An upcoming change will add another library that needs to be
added for backwards compatibility. Merging the tests for those into the
existing test class makes it much harder to see and the tests start to
overlap, i.e. some tests will test more than one aspect which makes
maintenance more difficult and debugging more complex.

This splits the test methods in PackageBackwardCompatibilityTest out
into separate tests for the different PackageSharedLibraryUpdater
implementations into their own classes and simply tests that the
PackageBackwardCompatibility class uses the correct implementations.

This allows each PackageSharedLibraryUpdater to provide comprehensive
tests for their own behavior without affecting tests for the other
classes.

The OrgApacheHttpLegacyUpdaterTest only runs if the
OrgApacheHttpLegacyUpdater class is on the classpath. That is done
using OptionalClassRunner which is a custom JUnit Runner that runs the
tests in a class iff a specific class is present. Otherwise, it behaves
as if the class had a single test that made an invalid assumption.

Tested by building with and without REMOVE_OAHL_FROM_BCP=true and then
running the following:
adb install -r -g out/target/product/marlin/testcases/FrameworksCoreTests/FrameworksCoreTests.apk &&
adb shell am instrument -w -e class android.content.pm.PackageBackwardCompatibilityTest,android.content.pm.AndroidTestRunnerSplitUpdaterTest,android.content.pm.OrgApacheHttpLegacyUpdaterTest,android.content.pm.RemoveUnnecessaryOrgApacheHttpLegacyLibraryTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner

Bug: 18027885
Test: as above
Change-Id: Idd1a343d234a57d518010c5a79030cbd7682e0c1