libcore: Clean up Enum.compareTo(E)

As E extends Enum<E>, it is safe to cast the comparable object to
Enum<?> as it should not throw any exceptions. When it is cast, the
ordinal value can be accessed directly. Doing so would be preferred as
either both the variables this.ordinal and o.ordinal should be used or
the methods this.ordinal() and o.ordinal() should be used without
mixing them together.

Additionally, this fixes a build issue for JDK7 as the compiler was
complaining about the ordinal variable being private.

Change-Id: I07766b2756eaf09de2b054932bebc39dccce5731
Add API to check certificate chain signatures

Add hidden API to check certificate chain signatures when needed. The
getCertificates implementation returns a list of all the certificates and
chains and would expect any caller interested in verifying actual chains
to call getCodeSigners instead.

We add this hidden constructor as a stop-gap until we can switch callers
over to getCodeSigners.

Bug: 13678484
Change-Id: I01cddef287767422454de4c5fd266c812a04d570