Use more fine-grained lock when persisting shortcuts to disk

Currently we are holding service-level lock whenever a process talks to
ShortcutService via any of the public apis, which unnecessarily
decreases the overall throughput.

At the time ShortcutService was implemented, all shortcuts from various
packages were persisted into a single xml file on a write-through basis.
This was the main reason a single synchoronization lock was required
since two parallel updates might corrupt the xml file.

Since that shortcuts from different packages are moved into separate xml
files, in this CL we:
1. Only write the shortcuts from individual packages into corresponding
   xml file after there's a change to the shortcuts in that package, as
   opposed to writing every shortcuts from every packages under that
   user into corresponding xml files, which was rather inefficient.
2. The write operation is still scheduled as a background job, but it no
   longer holds the service-level lock. Instead it would only hold
   package level lock since it is only writing into the xml from that
   specific package.

Given that file I/O is the most time-consuming task in ShortcutService,
refrain from holding service-level lock when performing these type of
tasks should help overall system throughput.

Bug: 186011943
Test: atest CtsShortcutManagerTestCases
Change-Id: I773f4c1d7a39e26030a78ee57f29bf0f33848614