docs: Updating obsolete "Key Character Map" link

The target page moved a couple of times and it looks like there wasn't a
redirect set up. Verified that
https://source.android.com/docs/core/interaction/input/key-character-map-files
currently works.

Change-Id: Ia3ea39ec7c2fd99c46bb7194790c999e1d9a35c4
Fixes: 224379018
Test: m -j 8 ds-docs-switched