Cache subtype lists reasonably.

This will spare a lot of IPC for Latin IME at the cost of very little
retained memory.
This improves the loading by potentially a lot - between 15 and 30%
when the layout is cached (which should now be the case almost every
time), and half that if it's not. More importantly, it makes the
load time less sensitive to high device load, which is one of the
sore points.

Bug: 8689779
Change-Id: I2e07736f1a92c38eed0e203bc690761a181da8b9