[AppBar] Move shadow layer initialization back to the init phase. (#4746)
[AppBar] Move shadow layer initialization back to the init phase. (#4746)

This logic was moved to the app bar view controller's viewDidLoad as part of 2645cc539f6a17e8ecb729a1b5b7997be24f07fa. However, this affected some downstream clients that were depending on the shadow layer already being initialized immediately after initializing the app bar.

This change essentially reverts the moved behavior by moving the initialization to the app bar view controller's init- methods.
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios