WM/ViewRootImpl: Fix convertFromTranslucent to respect existing state.

Code-paths triggered by convertFromTranslucent end up setting an opaque
state both on the client side and the WindowManager side, without
appropriately checking to see if we were already non-opaque before we
were convertedToTranslucent.

Bug:126897750
Test: Manual
Change-Id: I2d19624be31330100b61615fabae04899d95cda9