Refactor ProtoLogController for testability and add unit tests

Made ProtoLogController more amenable to unit testing by extracting
initialization logic into protected methods.

Added ProtoLogControllerTest.java with comprehensive unit tests
covering group registration, initialization paths, and state
management. This addresses the TODO for dedicated controller tests.

Test: atest TracingTests
Flag: TEST_ONLY
Change-Id: If84fdbcc286e646e9913267698c99c1b23630e26