Make CombinedVibrationEffect constructor package-private

Also add @SuppressWarnings to this class to allow it to be abstract,
since it's only extended within the framework codebase.

Fix a missing @NonNull annotation found after building the code without
the @hide annotations.

Bug: 166586119
Test: atest CombinedVibrationEffectTest
Change-Id: I1b283c004efb7fb023099fb7f4db455b9994d600