Use Gradle runner step instead of Shell script at bitrise.io

[vgaidarji]

Build on bitrise is failing due ANDROID_NDK_HOME not being properly unset.

The temporary fix would be run build commands via shell step as

unset ANDROID_NDK_HOME

./gradlew assembleDebug testDebug jacocoTestReport checkstyle pmd jdepend lintDebug buildDashboard assembleDebugAndroidTest crashlyticsUploadDistributionDebug -PversionCode=$BITRISE_BUILD_NUMBER -PfabricApiKey=$FABRIC_API_KEY -PfabricApiSecret=$FABRIC_API_SECRET

As per Krisztián Gödrei (godrei)

the problem is that steps are running in separate shell sessions, so a step can not modify the parent's environemnt, thats why we introduced envman. But envman has no feature to unset envs, envman can only clear envs, but it turned out that it is not enough, the ANDROID_NDK_HOME needs to be fully removed from the env set

No ETA when it's going to be fixed on bitrise side. Will need consider whether it makes sense to have workaround implemented or wait for official fix from bitrise.

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Original issue #27. Will leave this one open to move back to Gradle runner step when bitrise properly unsets ANDROID_NDK_HOME variable in their stack (bitrise-steplib/steps-gradle-runner@2f330b7).