Handle package broadcasts before apps do

- Have PM to call the shortcut manager directly before sending a
pacakge broadcast.

- The shortcut manager will enqueue a task to handle a package
broadcast, which will be executed on Handler.

- At the entry points of all external facing methods, block until
all pending tasks are finished.

Bug 29895275

Change-Id: Ib29fa3c9c8d9b9e0ca5c8f1e9da2a390324960a4