lmkd: Protect against buffer overflow

We're passing a 'line' whose backing buffer is PAGE_MAX in size
into memory_stat_parse_line().  We protect overflowing the smaller
LINE_MAX 'key' buffer via some C preprocessing macros to assure
we limit the size.

Test: Local build with LMKD_LOG_STATS set for this file.
Bug: 76220622
Change-Id: I9e50d4270f7099e37a9bfc7fb9b9b95cc7adb086