Remove dead Api24Impl in accessibility delegate

Compose UI requires minSdk >= 24, making the
Build.VERSION.SDK_INT >= Build.VERSION_CODES.N check guaranteed true.
This CL inlines addSetProgressAction and removes the dead Api24Impl
wrapper object.

Test: Existing AndroidComposeViewAccessibilityDelegateCompatTest
Change-Id: I4f8e8ba6669d31c03eb5e8964f3d5f1217424963