On my system (Win 10, JRE 1.8.0, Android Studio 3.1.2, Gradle 4.1) there is no tools.jar in the JRE directory (C:Program FilesJavajre1.8.0_171).
However, I found it in C:Program FilesAndroidAndroid Studiojrelib and tried setting JAVA_HOME=C:Program FilesAndroidAndroid Studiojre