wtf() should not be fatal for core components.

Core system components (such as those marked as "persistent") are
aggressively restarted by the system, so crashing them as a side
effect of wtf() could easily cause system instability.

Instead, this change now treats persistent processes the same as the
system_server; we still log the wtf() event, but we don't crash
the app.

This change also causes wtf() events to be fatal on "eng" builds,
which will result in bugs being caught earlier during development
instead of later during QA testing.

Test: builds, boots, SystemUI no longer crashes
Bug: 32976626
Change-Id: Ib2d1a73379be40556f8dab5d1f15b9ed91ba7082