Add sanity check when loading public keys for OTA package

For RSA keys, check if it has a 2048 bits modulus, and its public
exponent is 3 or 65537. For EC keys, check if the field size is 256
bits for its curve.

Bug: 116655889
Test: unit tests pass
Change-Id: I5c00f4d2b61c98c434f0b49db232155d5d0770ec