Android CTS 7.0 Release 32 (5731995)
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Update harfbuzz to 1.2.6 from 1.2.1

Chromium moved to 1.2.6 in ToT and it's recommended by
the upstream (Behdad).

For the acutal changes, see

https://android.googlesource.com/platform/external/harfbuzz_ng/+log/dabf32a5ad..00ea66

There's no change in 'rl' files so that there's no change in the
corresponding generated header files, either.

Android.mk was updated along with src/hb-version.h and README.version.

BUG: 28006027

Change-Id: I96068caae7b98d5665b0c71918f1b50deb428e2b