Get WindowManager only when needed

 Do not keep any WM references in ToastPresenter, only get on demand.

Test: atest WindowUntrustedTouchTest
 atest ToastWindowTest
 atest ToastPresenterTest
 atest ToastUITest
 Manual run: the stress-test app attached in the bug

Bug: 309562471
Change-Id: I30d048b01c0890d2fff3bc7684a20defbf589ffb