Bump mockito-core from 2.19.1 to 2.21.0 (#974)

Bumps [mockito-core](https://github.com/mockito/mockito) from 2.19.1 to 2.21.0.
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](https://github.com/mockito/mockito/compare/v2.19.1...v2.21.0

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

mobile/build.gradle

@@ -130,7 +130,7 @@ dependencies {
     annotationProcessor 'com.google.auto.value:auto-value:1.6.2'
     annotationProcessor 'com.ryanharter.auto.value:auto-value-parcel:0.2.6'
 
-    testImplementation 'org.mockito:mockito-core:2.19.1'
+    testImplementation 'org.mockito:mockito-core:2.21.0'
     testImplementation 'junit:junit:4.12'
     testImplementation 'org.json:json:20180813'
     testImplementation 'com.squareup.okhttp3:mockwebserver:3.11.0'