Add 'pm' operation to set a package's app-linking state

Set an app's state:

  pm set-app-link [--user USER_ID] PACKAGE {always|ask|never|undefined}

Read an app's current state:

  pm get-app-link [--user USER_ID] PACKAGE

The latter prints to stdout one of the strings usable as an argument to
set-app-link.  If an error is encountered, the string printed to
stderr begins with "Error: ".

Bug 19628527

Change-Id: I68b6dc24445917807345a8cf5baa2078490740af