Recovery now expects public keys in zipfile

This is in line with the build system change which copies the recovery ota
install keys to a zipfile. And now recovery will parses and loads the public
keys from /res/otacerts.zip. The legacy load_keys functions will be
removed in later cls.

Bug: 116655889
Test: sideload an ota package
Change-Id: I95e91736ca9964df06d74aa292d672e2f9e442e8