To reproduce:
1. Open attached project "655009_SignGradle.zip"
2. Make sure keystore path is set with "PlayerSettings.Android.keystoreName = keystorePath;" in script SetKeystone.cs
3. Change platform to Android and build the project
4. Observe the build fail

Expected: provided keystore path is converted to full path
Actual: string with relative path is not converted to full, resulting in failed build (check build.gradle)

Reproduced in: 2019.1.0a12, 2019.1.5f1, 2019.2.0b5, 2019.3.0a5
Not reproduced in: 2017.4.28f1, 2018.4.1f1, 2019.1.0a11

Note1: setting keystore path via script on Windows results in keystore not being found due to different path string formatting
Note2: contrary to Windows, using GetFullPath() returns relative path in Mac's path field (however in built project, global path is included, therefore it works)