Update `AnalyticsBase` class access level to non-public

### Expected Behaviour
Unless we want users to be able to directly inherit from AnalyticsBase to create their own implementations, we should update the access level of the `AnalyticsBase` class to its default `internal` (removing the current `public` modifier) to reduce implementation confusion of which Analytics-type class to register.

### Actual Behaviour

### Reproduce Scenario (including but not limited to)

#### Steps to Reproduce

#### Platform and Version

#### Sample Code that illustrates the problem

#### Logs taken while reproducing problem

Find the AnalyticsBase class declaration in the Swift SDK and inspect how its access level is currently defined and used. Update the declaration to the default internal visibility, then run the existing test suite to confirm the SDK still builds and its tests pass.

Written by the indexing model from the issue text.