Split bettertogether test case into router and session test cases

Created two new directories (session and router) and moved the relevant files into each. Then updated package names to point to the right places.

Bug: 349106078
Test: Run presubmit and validate that all of the tests are still passing in both new test cases.
Flag: EXEMPT refactor
Ignore-AOSP-First: changes in topics with internal-first/internal-only projects

Change-Id: I76c269292d5dae28e6f8dd88c91321e3c05da57a