Camera: Restructure default camera.provider 2.4

To allow for implementation inheritance of @2.4 legacy wrapper and
@2.4 external webcamera HALs in the @2.5 implementations, restructure
the existing default provider to separate the service interface into a
thin shim that calls the implementations.

Test: Camera starts as usual after refactor, VTS tests pass
Bug: 121379978
Change-Id: Id40790ed4fb495577fd2b885c706b2ed7a96d64e