Fix the missing initialization of IP checksum

Remove using extra byte array for sending data

Remove some warnings

Version down to be compatible with Roboletric 4.3

Migrate to AndroidX

Version up ConstraintLayout

Remove build tool version

Version up the Gradle and its plugin

Merge remote-tracking branch 'origin/master'

Fix for failing to save TCP segment which size is over than 65356 size

Resolve the network is stuck

Add LICENSE

some refactoring

Change debug message correctly

Add graceful TCP termination and remove useless TCP window full check