Move ConditionalModifiers.kt to PlatformCompose Core

The Modifier.thenIf(condition) { ... }  was moved to
PlatformComposeSceneTransitionLayout in ag/25017678, in order to make
that library fully self-contained. After reflection, PlatformComposeCore
makes more sense for this file and it's fine to make
PlatformComposeSceneTransitionLayout depend on PlatformComposeCore
(which was done in ag/29200580).

Bug: 375119446
Test: presubmits, pure move.
Flag: EXEMPT pure move.
Change-Id: Id5012d3124e9ad7dda319e5cb16b0e5b5b7c9fe1