This CL instroduces WMS#attachWindowContextToWindowToken
and the corresponding API in WindowContextController to make
WindowContext able to associate with a WindowToken.
Test: atest WindowContextControllerTest
Test: atest WindowManagerServiceTests#testAttachWindowContextToWindowToken*
Bug: 159767464
Change-Id: I807a67fba149cbdc4267442bab0b64e6a95bd4b5