Fix build issues

[sergejsha]

This PR fixes issues described in #23

> Task :androidApp:mergeExtDexDebug FAILED
    AGPBI: {"kind":"error","text":"java.lang.OutOfMemoryError: Java heap space","sources":[{}],"tool":"D8"}
        com.android.builder.dexing.DexArchiveMergerException: Error while merging dex archives: 
        ...

Execution failed for task ':shared:compileDebugKotlinAndroid'.
> 'compileDebugJavaWithJavac' task (current target is 1.8) and 'compileDebugKotlinAndroid' task (current target is 18) jvm target compatibility should be set to the same Java version.
  Consider using JVM toolchain: https://kotl.in/gradle/jvm/toolchain
  ...