Separate BAL code from source

The current BAL code mixes the reason why BAL is allowed with the source
(caller/real caller) that allows BAL. This CL separates it for internal
purposes (i.e. logging), but keeps returning the original code. Also
until extra fields are added in the metrics it keeps logging the
original codes.

Test: atest BackgroundActivityLaunchTest
Bug: 306059525
Change-Id: I04884ec426b7a4b5ed7d13ef01e5f233a8c0f8f3