android: Fix crash when typing characters with accents

When typing in a European language (e.g. French), keys like '^' can
activate character compositions to type characters with accents.
When this happens, the unicode integer returned by getUnicodeChar() is
not a valid unicode code point.

BUG=450748221

Change-Id: I4a36854745cfb1bfe67c6a76ebd19f18d8807fb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7038962
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529489}