In ShortcutService, save data of each package in a separate file

This CL splits the main Xml file into individual files for each package.
This enables us to gain disk I/O performance improvements by selectively
updating only the parts that have been changed (coming in future CLs).
The old implementation was re-writing the entire shortcut data of a user
on every change.

Bug: 153680823
Test: atest com.android.server.pm.ShortcutManagerTest1
            com.android.server.pm.ShortcutManagerTest2
            com.android.server.pm.ShortcutManagerTest3
            com.android.server.pm.ShortcutManagerTest4
            com.android.server.pm.ShortcutManagerTest5
            com.android.server.pm.ShortcutManagerTest6
            com.android.server.pm.ShortcutManagerTest7
            com.android.server.pm.ShortcutManagerTest8
            com.android.server.pm.ShortcutManagerTest9
            com.android.server.pm.ShortcutManagerTest10
            com.android.server.pm.ShortcutManagerTest11
Change-Id: I3a29b80ecfa353eddb1bb778eb845d863f057e0a