external/icu: Change flexible array members to be of length 1 instead of 2

...so that -fsanitize=undefined does not report false out-of-bounds accesses;
Clang's isFlexibleArrayMemberExpr (lib/CodeGen/CGExpr.cpp) only treats arrays of
length 0 and 1 as such special flexible cases.

There appears to be no code in icu that depends on those arrays to be of length
2 (e.g., via sizeof), though it does look suspicious that they are deliberately
of length 2 instead of 1.

Change-Id: I85293e769f1d64cb4e60e13f1cd7f88b76e37487