Move Grids.kt back to PlatformComposeCore

The HorizontalGrid and VerticalGrid (non-lazy grids) were 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: I3218a78e811ea8070dec99db52bb231b7689215c