Update getSigningCertHistory docs to reflect signers returned in order

SigningInfo#getSigningCertificateHistory should return the signer's
in an APK's signing lineage in order of rotation. This commit updates
the API's documentation to explicitly state this requirement.

Bug: 193834256
Test: Build
Change-Id: I5c1f028cd12039cbbac9f6f6c77122639f8e1d9c