Upgrades the ICU version to 64.2

[filmil]

The current version is 62.1, which is a now 3 major releases behind,
given that ICU 65 was released on Oct 3, 2019.

The version bump required an update to buildroot to fix too strict compilation
of ICU.

Previous update was here:
#6097

I was advised to upgrade in the code review for this PR:
#13045

This change brings in the function icu::Locale::forLanguageTag(...)
which is directly relevant to the work in the PR, but also updates
the library considerably.

Tested:

set -x

readonly FLUTTER_ENGINE_DIR="${FLUTTER_ENGINE_DIR:-$HOME/fx/flutter/engine/src}"
readonly OUT_DIR="${FLUTTER_ENGINE_DIR}/out"

(
  cd ${FLUTTER_ENGINE_DIR}

  ./flutter/tools/gn --unoptimized
  ninja -j 100 -C "${OUT_DIR}/host_debug_unopt"
  ./flutter/testing/run_tests.py --type engine
)

[filmil]

@chinmaygarde @iskakaushik FYI

I'll probably need some good advice on how to test this stuff out.

[filmil]

There was an intervening update to the licenses files in the meantime. Rebased, and will use the diff report from https://github.com/flutter/engine/tree/master/tools/licenses to merge the licenses correctly. I don't think I can do this manually.

[filmil]

@chinmaygarde All precommit hooks have passed. Is there anything else I need to do here?