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
Don't account buffer.arrayOffset() twice.

data[OUTPUT_OFFSET] from the perspective of native
code must always be the offset from the start of the
buffer passed in to it.

Change-Id: I4a5c9595aebd105c57ce2239dc95cf85d5bd6256
Merge branch 'kk4.4' of https://github.com/SlimRoms/platform_libcore into kk4.4