[WIP] Preparation for a JCenter sunset (and switch to a Maven Central); also: android gradle plugin is bumped

[dejan2609]

Details / rationale:

• JCenter repository sunset is announced for a May 01st 2021 (and hence references must be changed to a Maven Central)
• 'com.android.tools.build:gradle' plugin version is bumped: 4.0.0-->> 4.1.2 (ℹ️ please note that bump to an upcoming version 4.1.3 (that will solve issues with gradle's dependencies refresh) is required, see pending issue here: https://issuetracker.google.com/issues/179291081).

At the moment Gradle's dependencies refresh failes: just open any module as a separate project in Android Studio and press Sync project with Gradle files OR use Gradle directly from a command line:

cd BasicRxJavaSample
./gradlew --refresh-dependencies

and you will receive error related to trove4j not being available to fetch (this specific trove4j version was published to a JCenter exclusively):

> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find org.jetbrains.trove4j:trove4j:20160824.
     Searched in the following locations:
       - https://dl.google.com/dl/android/maven2/org/jetbrains/trove4j/trove4j/20160824/trove4j-20160824.pom
       - https://repo.maven.apache.org/maven2/org/jetbrains/trove4j/trove4j/20160824/trove4j-20160824.pom
       - https://oss.sonatype.org/content/repositories/snapshots/org/jetbrains/trove4j/trove4j/20160824/trove4j-20160824.pom
     Required by:
         project : > com.android.tools.build:gradle:4.1.2 > com.android.tools.build:builder:4.1.2 > com.android.tools:sdk-common:27.1.2

Related links:

• https://jfrog.com/blog/into-the-sunset-bintray-jcenter-gocenter-and-chartcenter
• https://www.infoq.com/news/2021/02/jfrog-jcenter-bintray-closure
• https://blog.sonatype.com/dear-bintray-and-jcenter-users-heres-what-you-need-to-know-about-the-central-repository

[dejan2609]

Update: not relevant anymore, hence closing 🧹