Add install reason

This CL allows a reason to be specified when installing a package. The
install reason is a sticky piece of metadata: When a package is e.g.
installed via enterprise policy and an update is then manually
installed or sideloaded, the install reason will remain "policy."
The install reason is tracked separately for each user.

With this CL, two install reasons exist: "policy" and "unknown." Other
install reasons will likely be supported in the future.

Bug: 32692748
Bug: 33415829
Test: Tested manually with "adb install" / "adb uninstall"

Change-Id: I0c9b9e1b8eb666bb6962564f6efd97e41703cd86