You can put emojis in an XML, and the squares are just the editor's way to render characters it can't recognize, like all other characters, you can specify the character by its UTF8 value instead.
For example,
has the char value of 128515, so you could use 😃
to specify it. It is very similar to how you need to use <
to specify a literal '<' in an xml.
HOWEVER, as of writing this answer, using these tokens in (for example) strings.xml as string constants will cause your app to crash, with an error going all the way down to the JNI level of your android application (C++ code).
JNI DETECTED ERROR IN APPLICATION: input is not valid Modified UTF-8: illegal start byte 0xf0
I am not qualified to say how much this can limit your use of emojis in android in general, but I can say for certain that you can't add emojis to android xml files (without making your app crash, that is).