Rename SystemUIComposeCore to PlatformComposeCore (1/2)

This CL renames the SystemUIComposeCore library to PlatformComposeCore
and moves all com.android.systemui.compose.* files to
com.android.compose.

The files will still be in
frameworks/base/packages/SystemUI/compose/core/ after this CL, and I
will move them in a follow-up CL.

Bug: 263851158
Test: Builds & presubmits
Change-Id: I31776ae19de7f64f7697f3b4af55f2d97c3fc733