Add flag to cmd for dynamic services

When cmd is called with the -w flag, it will call ServiceManager::waitForService instead of checkService. This will make dynamic services start and block until they are ready for use.

Bug: 149526916
Test: atest ApexRollbackTests (with and without this flag applied within for dynamic apexd)
Change-Id: Iad0c3d7aa75e5830da10a84bd753d3011438259f