Fix testCreateWindowContextNewTokenFromClient

Since we change #detachWindowContext as an one-way method,
and the WindowToken may have not been detached yet,
using a pollingCheck to prevent the timing issue.

Test: atest WindowContextTest
Fixes: 409577481
Flag: EXEMPT bugfix
Change-Id: Ib6eaa58faa3d68bbb5d85439af5f6389af042d65