Don't insert dead key glyphs repeatedly

At the moment, holding a dead key will result in the insertion of
multiple characters. However, it's likely that was not the user
intention, so this CL addresses that.

Test: Built and tested on a test device
Bug: 266536908
Change-Id: I10b2d1865a3e789a3ee7c27a0358bfc8b9bd4d71