Thread.uncaughtExceptionPreHandler: temporarily graylist.

Blacklisting uncaughtExceptionPreHandler in Android Pie broke a major
third party app's use case. While it's too late for Pie, this CL
moves the field back to the greylist for Android Q to reduce impact
for that app.

We still plan to blacklist this field again in a future version
of Android after we've had a chance to consider whether that app's
use case should be addressed by a public API.

Bug: 133227292
Test: Treehugger

Change-Id: I21f5bf16ed34178ae06b63e41adf84285ce8d39f