[Decor] Split RoundedCornerResDelegate into impl and interface

In order to support a new debug corner res delegate, we should split out
the RoundedCornerResDelegate into an interface + implementation. This
will allow us to create a DebugCornerDelegate that can be reused by the
RoundedCornerDecorProvider without needing to duplicate that class
definition.

Test: RoundedCornerDecorProviderFactoryTest
Test: RoundedCornerResDelegateTest
Test: adb shell setprop debug.screenshot_rounded_corners 1 && restart-sysui
Bug: 285941724
Change-Id: I68fbe1a7d0662781b611c84ae81dd526381aad22