What is the initial TCP Receive Window size in Android?
I can answer you about TCP WINDOW SIZE and TCP WINDOWSCALE fields as they appear in the TCP header.

Initial TCP Window Size should be 0x3908 = 14600, at least it is what I found so far using several versions.

(On most Linux distro I've used it is exactly the double 0x7210 = 29200.)

Window Scale has different values depending on the version, older set it to 6, newer set it to 8, at least for what I experienced so far.

You can check both values with this app:
https://play.google.com/store/apps/details?id=be.ac.ulg.mobiletracebox 
It will show you them in the first line 

0: youraddress ... TCP::Window (0xXXXX) ... TCP::WindowScale (0xXX)

Values are hexadecimal. 

For Window Size use regular Syn probe.

For Window Scale use Window Scale probe.

(If no value is shown for Window, it should be the max (0xffff=65535). If your device is rooted you should set mode to server based.)

PS
I don't know if talking about an app is OK here, if not I'm more than willing to edit my comment.