Include OrgApacheHttpLegacyUpdater unconditionally

Previously, OrgApacheHttpLegacyUpdater was added to the bootclasspath
(in the framework-oahl-backward-compatibility library) if and only if
the org.apache.http.legacy.jar was not on the bootclasspath. Its
presence caused the runtime to maintain backwards compatibility by
adding org.apache.http.legacy.jar to the app's classpath for apps that
target < P.

The org.apache.http.legacy.jar was permanently removed from the
bootclasspath in P and so the framework-oahl-backward-compatibility
library, and hence OrgApacheHttpLegacyUpdater, are always on the
bootclasspath and so there is no need to treat it as optional. To that
end this change:
1) Includes it in the main framework target, previously it was
   explicitly excluded.
2) Removes the code for handling the case when it is not present.
3) Removes tests for when it is not present.
4) Removes documentation describing what happens when it is not
   present.

Bug: 18027885
Test: cts/tests/signature/runSignatureTest.sh
Change-Id: I3892ead8cd2a8bf17e57bf08e3bf6ee65afac0a1