Extend update-tzdata.py to generate ICU overlay

Extend update-tzdata.py to generate ICU overlay time zone
.dat file (a .dat file that just contains time zone data and can be
used to extend a full .dat). Delete createIcuUpdateResources.sh
since this replaces that script.

Tested with:

Ran update-tzdata.py, compared resulting
system/timezone/output_data/icu_overlay/icu_tzdata.dat file with output from
./createIcuUpdateResources.sh $ANDROID_BUILD_TOP/system/timezone/input_data/iana/tzdata2017b.tar.gz 58

Bug: 31008728
Test: See above
Change-Id: If92062c2ec6a16b12affe61b4cd9bcf8f6861afd