Add a unit test for isSealed()

Bug: 177424937

The Watchable* arrays in server/utils have an isSealed() method.  Add
a unit test for the method.

Test: atest
 * FrameworksServicesTests:WatcherTest

Change-Id: I3a2e5bd82e220f1a253065023d95aa927421b97b
1 file changed