Insets: Fix DecorView consuming of insets

Previously, disabling the consuming logic was
done based on whether we target R.

But this:
- didn't work for Q apps that already use the new API
- causes a bit of unnecessary behavior churn
- requires special-casing the IME

Instead, we look at whether the app has requested to
handle insets with the R setDecorFitsSystemWindows API.

Fixes: 152384150
Test: atest DecorInsetSdk29Tests DecorInsetTests
Change-Id: I754851d0c11dccd21c2fa011cfdc3f0d7396cf60