MediaSession2: Remove callback proxy interfaces

This is refactoring CL for better test readability.

Previsouly MediaSession2TestBase#createController() takes
TestControllerCallbackInterface, and it wasn't natural to understand
why such interface is needed instead of ControllerCallback. For better
readability, removed similar callback proxy interfaces. Here's the list.
  - TestControllerCallbackInterface
  - TestBrowserCallbackInterface
  - SessionCallbackProxy

Bug: 74424357
Test: Run session2 tests in CtsMediaTestCases
Change-Id: I5eb62f219940003749fbe2bc6443c6b56f8fa210
8 files changed