Allow null initial state in SavedStateRegistry constructor

Make initialState parameter nullable in SavedStateRegistry constructor.

RelNote: "Allowed null initial state in SavedStateRegistry constructor."
Bug: 548538186
Test: ./gradlew :savedstate:savedstate:testAndroidHostTest :savedstate:savedstate:desktopTest
Change-Id: I76006f21477e43185e79a01753d52a89d45b34af