lmkd: Add support for process death notifications

With pidfd polling support lmkd can detect process death without periodic
polling. Implement mechanism to detect kernel pidfd support using
pidfd_open syscall existence as an indicator. Implement the logic to use
pidfd to wait for process death.

Bug: 135608568
Test: lmkd_unit_test with and without pidfd kernel support
Change-Id: Ic6db7e50893534467f5130a7f998b66fb4451272
Signed-off-by: Suren Baghdasaryan <surenb@google.com>