Do not return null in getSlice()

- It will result in a empty SliceView. Because we can use
slice.hasHint(HINT_ERROR) to check if we should show the slice,
always return a valid slice object.
- Set tintcolor -1 therefore battery icon will not have tint

Bug: 120221527
Test: manual
Change-Id: Idfc97f90c75515b079e1faed802c9b9047bfdeb4