Don't dump stack traces for background ANRs.

Dumping stack traces can be extremely expensive, and doing so for
background applications often has extremely negative side effects for
foreground applications. This can be exacerbated by resource-intensive
applications, because those may exhibit thermal throttling in the first
place. For such applications, the additional performance hit caused by
stack dumps may be catastrophic.

Instead, don't dump stack traces for background ANRs except for the app
that actually ANR'd.

bug 30112521

Change-Id: I8a05059343254861c436a193690cd1c50a95d674