Concise toString for A11yEvent

I found A11yEvents hard to parse in logs as they take ~3 lines on my
30" monitor, most of which denoting fields that rarely differ from default.

This CL adds an opt-in debug flag to not print default-valued fields, which
dramatically cuts down the noise when trying to parse logs.

Test: enable DEBUG in A11yManager, and observe logcat
Change-Id: I979200194ee3597c1a8501195437b4d4b8e21514