E2e tests: Add freeform check when waiting for enter-desktop transition

When entering Desktop mode: instead of just waiting for a transition to
finish, with this CL we wait for the app we're moving to Desktop to be
in freeform mode.

Also make sure when we try to move an app to Desktop through the app
handle menu, that we actually find the Desktop button in the app-handle
menu.

Bug: 421118232
Flag: EXEMPT e2e test fix
Test: atest WMShellFlickerTestsDesktopMode
Change-Id: Ifdcda8e13883fd692e4730ddf2955df0b6ed8e81