Get the instance count as the initial value

Developer may not be able to enable strict mode before creating any
activities. The initial value of expected instance count may not always
be 1.

This CL use the instance count from InstanceTracker as the initial
value.

Bug: 152348723
Test: atest ActivityLeakTests
      atest NexusLauncherTests
Change-Id: I8d34c87748deac72cf8b6e5cc247029901bbffb3