Refactor CustomSliceManager.

- Make everything that can be static, static.
- Also removed a unused slice for battery info.

Next step - going to make CustomSliceManager into a regular class
instead of getting it from FeatureProvider. This way it's no longer an
application level singleton, then we can do things like attaching
lifecycle to it. It will be easier to deal with "session" based use case
after the refactor

Bug: 123937830
Test: manual
Change-Id: I384761a6894fa907f72a3610437835c4dd4d1944