logd: test: switch to /data/backup/ for sepolicy avc injection

Resolve issues seen on continuous testing frame:

- statistics test, info instead of fail on missing radio log data.
- sepolicy switch from /data/misc/logd/ to /data/backup/ as the
  directory we access(2) to inject sepolicy violations.  The key here
  is we are still root, but we are in u:r:shell:s0, and the directory
  does not provide us DAC access (0700 system system) so we trigger
  the pair dac_override and dac_read_search on every try to get past
  the message de-duper.  /data/misc/logd is not always there, until
  logpersist is enabled, but /data/backup is always there.
- a stricter signature of '): avc: denied'
- put in a looser threshold for sepolicy_rate_limiter_spam test.

Test: gTest logd-unit-tests --gtest_filter=logd.sepolicy*
Bug: 34454758
Change-Id: I28ce4fdb51dc4869944e3253b593ce222d16ec98
1 file changed