Use the same fixed channel allocator for both Classic and LE connections

* Merge identical logic for fixed channel allocation among Classic and
  LE fixed channel connections
* Use Classic mocks as a place holder to write tests for
  L2capFixedChannelAllocatorTest

Test: bluetooth_test_gd
Bug: 140938432
Change-Id: Ibff6cb2c3120d967efe828e91cb8f9f2bdefc398