tombstoned: change path for traces from "anr_" to "trace_"

The only case where tombstoned creates files for java traces is
when the process is signalled "by hand" using "shell kill -3", or
by the program itself. Such traces do not correspond to an ANR, so
name those files "trace_XX".

When dumpstate / system_server want to dump java traces, they set up
a tombstoned intercept and manage the lifetime of any associated file
that themselves.

Bug: 32064548
Test: manual, debuggerd_test
Change-Id: I97006ec7c0cd35de4b9564f535e77af846cc3891