Unable to resolve target 'android-7'
解决方案：
修改工程目录下的default.properties文件里的内容target=android-7改成target=android-12(或者其他版本)就可以了，最好用txt打开，然后修改之后保存，运行就可以了。
原代码如下：
# This file is automatically generated by Android Tools.
# Do not modify this file -- YOUR CHANGES WILL BE ERASED!
#
# This file must be checked in Version Control Systems.
#
# To customize properties used by the Ant build system use,
# "build.properties", and override values to adapt the script to your
# project structure.
# Project target.
target=android-12