Log APK hash for package install attempts in the Event Log.

This CL adds a package_digest field to the install_package_attempt
event. The field is populated with the SHA-256 digest of the contents
of the APK iff the user has consented to app verification and app
verification is enabled.

Bug: 10605940
Change-Id: I0d191398ed8d28950c7b5ee0ce02ec077d2c888b