Avert a crash in the Android last resort font driver

Avert a crash in the Android last resort font driver

* java/org/gnu/emacs/EmacsFontDriver.java (FONT_INVALID_CODE):
New constant.
(hasChar, encodeChar): Accept a jint character code rather than
a jchar.

* java/org/gnu/emacs/EmacsSdk23FontDriver.java (hasChar)
(encodeChar): Return 0 or FONT_INVALID_CODE if the character is
not representable in a Java char.

* java/org/gnu/emacs/EmacsSdk7FontDriver.java (hasChar): Ditto.

* src/androidfont.c (android_init_font_driver): Adjust signature
to match.