Add @SystemApi to OverlayManager that ThemePicker needs

Test: builds and tested in local theme picker

Bug: 121328713

Commands executed:
$ make system-api-stubs-docs-update-current-api
$ make api-stubs-docs-update-current-api

Cts tests to follow

Change-Id: Id26d32f482c1bbab3497b517b7a553d145a1e3df
Signed-off-by: Hyunyoung Song <hyunyoungs@google.com>