lmkd: Disable memory.stat usage when per-application memcgs are not used

Per-application memory.stat files are not available when per-application
memcgs are not used (per_app_memcg=false). Disable its usage based on
ro.config.per_app_memcg property.

minchan:
* correct indentation of memory_stat_parse
* move per_app_memcg check into memory_stat_parse inside
* change low_ram_device to per_app_memcg

Bug: 110384555
Test: manual test to see lkmd log message with memory hogger
Merged-In: Ib6dd7586d3ef1c64cb04d16e2d2b21fa9c8e6a3a
Change-Id: Ib6dd7586d3ef1c64cb04d16e2d2b21fa9c8e6a3a
Signed-off-by: Minchan Kim <minchan@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>