Move bitmap saving into corresponding ShortcutPackage

Currently saving the bitmap files associated with a shortcut still
requires holding service-level lock, this CL replaces that with
package-level lock to increase overall system throughput.

Bug: 186011943
Test: atest ShortcutManagerTest1
Test: atest CtsShortcutManagerTestCases
Change-Id: Ibac24eea6facb4221b6bc3e92c8130fe260c7df4