Cache the scaled avatar drawables in the keyguard user switcher

Loading the avatar icons and drawing them into the sized bitmap
turns out to be quite expensive and the cost increases with number
of users. Caching them shaves off several hundred milliseconds from
Keyguard inflation time during user switching on the lockscreen.

For instance, 15ms vs. 750ms with 3 avatars on a certain 7" tablet.

Bug: 7986933
Change-Id: I3e2065bfa25aa263133ba204ca364c3b04d7c0ff