Move publishing from JCenter to Maven Central

[samtstern]

Fixes #1910

TODO:

• POM generation
• Get rid of nasty centralized build
• Publishing instructions
• Check that publishing actually works

Note, this changes how dependencies that come from the Firebase BOM are reflected in the pom.xml, however this seems to be more correct based on the docs here:
http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#importing-dependencies

Before

  <dependencies>
    <dependency>
      <groupId>com.firebaseui</groupId>
      <artifactId>firebase-ui-common</artifactId>
      <version>7.1.1</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>com.google.firebase</groupId>
      <artifactId>firebase-database</artifactId>
      <version>19.5.1</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>androidx.legacy</groupId>
      <artifactId>legacy-support-v4</artifactId>
      <version>1.0.0</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>androidx.recyclerview</groupId>
      <artifactId>recyclerview</artifactId>
      <version>1.1.0</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

After

  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>com.google.firebase</groupId>
        <artifactId>firebase-bom</artifactId>
        <version>26.0.0</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>com.firebaseui</groupId>
      <artifactId>firebase-ui-common</artifactId>
      <version>7.1.1</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>com.google.firebase</groupId>
      <artifactId>firebase-database</artifactId>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>androidx.legacy</groupId>
      <artifactId>legacy-support-v4</artifactId>
      <version>1.0.0</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>androidx.recyclerview</groupId>
      <artifactId>recyclerview</artifactId>
      <version>1.1.0</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

[samtstern]

Goodbye to most of this rats nest!