manifest: Fixup: Update to freetype 2.6.0

The Freetype library in the Android source tree was outdated.
The old version had known security vulnerabilities and was updated
to 2.6.0 in AOSP master and tag android-m-preview-1.

Fix: Use refs/tags instead of just the tag name.

Change-Id: I941b970aa5d01c42c20579c8b92ddd5c18ee7a55
Ticket: CYNGNOS-677