Refrain from choking sysram due to excessive onShortcutChanged callbacks

When developers calls pushDynamicShortcut, LauncherAppService captures a
snapshot (i.e. cloned) of all of the existing shortcuts and passes them
through the callback. We observed an issue where, if the publishing app
decided to push a large quantity (say, 100+) of shortcuts in a for-loop,
we ended up generating a massive amount of shortcut instances (since the
shortcuts passes to the callback is a cloned instance) which leads to
a short burst in memory usage.

This CL debounces the callback by 100ms so that when the developers are
calling pushDynamicShortcut in a for-loop, callback will only be fired
for the last invokation.

Bug: 218545269
Test: manual
Change-Id: I9deca76d15fe9a71574c53031bd7aef7f7740740