Set CaptionController to null when views are released

Remove caption controller reference when views are released rather than
keeping the reference saved in WindowDecoration2.

Merges captionController#releaseViews and captionController#close into
one function since there is no point where one should be called but not
the other.

Bug: 437194304
Test: manual testing
Flag: com.android.window.flags.enable_window_decoration_refactor
Change-Id: I65340acfb1e32100e5f8cc1ab2909d2f7e62308d