update android-maven-plugin to version 3.8.0

I had the following error, that was solved by updating android-maven-plugin to 3.8.0.

[ERROR] Failed to execute goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.7.0:generate-sources (default-generate-sources) on project wallet: Execution default-generate-sources of goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.7.0:generate-sources failed: A required class was missing while executing com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.7.0:generate-sources: Lorg/sonatype/aether/RepositorySystem;