Add internal WM API for registering activity interceptors.

Each callback will map to a unique id. The id is stored in an enum, and we use the ordering of the enum to determine the order the callbacks should run in.

Test: locally on device
Test: atest WmTests:ActivityStartInterceptorTest
Test: atest WmTests:ActivityTaskManagerServiceTests
Bug: 191994709
Bug: 191996331
Bug: 200324021
Change-Id: I34a1e88852b51444fff751b25e138a44f45e1773