make nsUptimeMillis be uint32 on armv6

this means that millis will wrap in ~49 days, and nano in about 1 hour
which isn't ideal, but I can't see how else to deal with it.
AFAICT none of the in-tree users of renderscript use nano, and the
users of millis aren't affected by it wrapping.

Change-Id: I6ee03426753c6223599c5a963c79cc1959ed3b76