Move tzdata scripts from bionic to external/icu

The scripts affect files in external/icu, bionic
and the generated files affect libcore. The files must
be updated together so there is no "obvious" home.

OEM developers seem to want to update ICU
themselves and have been asking how. Moving the
scripts to external/icu and splitting the ICU generation
code into a sub-script they can run makes some sense.

Bug: 23419215
Change-Id: Ia26fa526fd2b560a79f36d327a10e262a85db752