Store ANR traces in internal directory

By storing ANR traces in /bugreports/dumptrace_XXXXXX,
dumpstate may unlink the ANR traces successfully to
avoid a storage leak.

Test: adb bugreport, ensure ANR traces are in the bugreport
      and that the temporary ANR traces file has been
      unlinked
Bug: 122292569
Bug: 179812900
Change-Id: I50e62d5dea1cb7d2aea316553eec310fb579e52d