add copy of public cert used for signing to OTA packages

When signing a file with -w (ie, an OTA package), add the file
META-INF/com/android/otacert, which is a copy of the public key
certificate.  While this can be extracted from the CERT.RSA file,
having a copy of it more easily accessible makes it easier to write
tools.

Bug: 6477365
Change-Id: I8cdb19536eca9a223c2b954e3f8ea0d9f3f86f02