lmkd: rate-limit and cleanup failed kill reports

Excessive number of failed kill reports when lmkd can't find an eligible
process to kill or frees not enough memory pollutes logs and bugreports.
Cleanup kill reports to remove duplicate information and rate limit failed
kill attempts at 1 report per sec. The number of suppressed failed kills
will be reported in the next lmkd report.

Bug: 113864581
Test: Verified using lmkd_unit_test
Change-Id: I67fa1fec97613f136c7582115edcbc56b1503c9c
Merged-In: I67fa1fec97613f136c7582115edcbc56b1503c9c
Signed-off-by: Suren Baghdasaryan <surenb@google.com>