Vastly reduce the number of read(2)s done in tzload.

This costs us about 1000 fewer syscalls, which makes "adb shell strace date"
a lot more readable (which is the reason I've been meaning to fix this for a
long time now), but also actually saves a measurable amount of time.

Longer-term we should try to keep the tzdata mmap(2)ed in like libcore
does.

Change-Id: I1dd9c81968a13d3a6a55ba17f8a7d5c1f38cd103