Fix inset rect calculation

The rect's |bottom| was mistakenly set to the height of the caption, but
it also needs to be offset from the rect's height, otherwise it could
end up with a bottom value lower than top, which results in a negative
height that is used to report insets.

Flag: com.android.window.flags.enable_window_decoration_refactor
Bug: 409648813
Test: check app content is offset to account for caption insets
Change-Id: I5ee787ebb66f776e5258bf76f2a09e111ba9e5e0