Restrict number of shortcuts can be added through addDynamicShortcuts

This CL fixes the issue where, when an app have multiple main
activities, the total number of shortcuts can grow indefinitely if they
were published through addDynamicShortcuts.

Bug: 281061287
Test: manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:3215e73e36aa0463429226b5743ce24badf31227)
Merged-In: Ib3eecefee34517b670c59dd5b8526fe9eb24f463
Change-Id: Ib3eecefee34517b670c59dd5b8526fe9eb24f463