To make it permanent on your system and the variable keep working after close the terminal, ou after a restart use:
nano ~/.bash_profile
Add lines:
export ANDROID_HOME=/Users/<username>/Library/Android/sdk
export PATH=$PATH:$ANDROID_HOME/platform-tools
export PATH=$PATH:$ANDROID_HOME/tools
export PATH=$PATH:$ANDROID_HOME/tools/bin
export PATH=$PATH:$ANDROID_HOME/emulator
Reopen terminal and check if it worked:
source ~/.bash_profile
echo $ANDROID_HOME