Make lmkd memory pressure levels configurable.

By doing so different platforms can customize them accordingly.
For instance a low mem, 512MB device can do
ro.lmk.medium=400, while a device with more memory can keep
the default(800).

Test: tested on gobo.
Bug: 64316084
Change-Id: Ifc4f3853bc06f30488adb25883ccd9aaf683ba9b