lmkd: Maintain pid to taskname mapping to amend kill reports.

Required because the kernel cannot always get the taskname safely at
the time the process is killed (due to competition for mm->mmap_sem).

Test: manually
Bug: 130017100
Signed-off-by: Jim Blackler <jimblackler@google.com>
Change-Id: I27a2c3340da321570f0832d58fe9e79ca031620b