Log code only in BAL log

The regular toString of the BalVerdict was extended to include the
source and already contained a lot of extra information if
DEBUG_ACTIVITY_STARTS was enabled.
Now this only logs the code in the public BAL log.

Test: atest BackgroundActivityLaunchTest
Flag: EXEMPT logging only
Change-Id: I666b5ba70d58afff1d17db6fd53492e9dd814585