whether implicit trusts such as for example Google Play Licensing[1] or explicit trusts such as for example the set of Certification Authorities Android devices ship with, you are trusting Google in many ways.
In the example you provide, you are trusting google in the same way you are trusting every SSL cert authority. What you are responding to is a reference to the signing of the APK, which is not (and cannot be) done by google even if the security of the transport layer is compromised.