Implemented the handling of custom perfetto configs.

Added the create_custom_config method in config_builder.py,
added the import and call to the create_custom_config method
in command_executor.py, and added 3 new unit tests and a
setUp method to tests/config_builder_unit_test.py to test
the create_custom_config method. Finally, made very minor
refactoring changes to the existing tests in
tests/config_builder_unit_test.py that include renaming of
the tests, moving a common line into the new setUp method,
and replacing common integers with constants.

Test: atest config_builder_unit_test
Bug: 357914823

Change-Id: Ic3436182a25a812b8766cb8f820d0c2f73406d0b
3 files changed