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: 11275004
Bug: 10605940

(cherry picked from commit 8fca480b)

Change-Id: I9773925f7e397ada26efac022349dc8e4af01208