Fixes an issue in shortcut persistence.

Currently after reboot the shortcuts previously saved in AppSearch could
be lost since ShortcutService never calls AppSearchSession#close.
Closing the session upon device shutdown fixes the issue.

Also, removes the exception where calling AppSearchSession#close gives
rise to IllegalArgumentException when the call wasn't made from a binder
thread. Since ShortcutService runs in the system process, it is expected
that calling AppSearchSession#close doesn't goes through binder.

Bug: 183982287
Test: flash, pin shortcuts, reboot, observe
Test: atest ShortcutManagerTest1 ShortcutManagerTest2
ShortcutManagerTest3 ShortcutManagerTest4 ShortcutManagerTest5
ShortcutManagerTest6 ShortcutManagerTest7 ShortcutManagerTest8
ShortcutManagerTest9 ShortcutManagerTest10 ShortcutManagerTest11
Test: atest CtsShortcutManagerTestCases

Change-Id: I61bc4354735770630965ca4f81314d23671165d0