Add Khmer layout

java/res/values-km/donottranslate.xml

java/res/xml-sw600dp/rows_khmer.xml

java/res/xml-sw600dp/rows_lao.xml

java/res/xml-sw600dp/rows_thai.xml

java/res/xml/kbd_khmer.xml

java/res/xml/keyboard_layout_set_khmer.xml

java/res/xml/method.xml

java/res/xml/rowkeys_khmer1.xml

java/res/xml/rowkeys_khmer2.xml

java/res/xml/rowkeys_khmer3.xml

java/res/xml/rowkeys_khmer4.xml

java/res/xml/rows_khmer.xml

...droid/inputmethod/keyboard/internal/KeyboardTextsSet.java

...-keyboard-text/res/values-km/donottranslate-more-keys.xml