Prefer mavenCentral

[jjliu15]

Description

Fork of #488, which was automatically closed when the main branch of the repo was renamed.

Adds mavenCentral() to android build.gradle files, for https://blog.gradle.org/jcenter-shutdown.

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[luo86]

Just would like to confirm - do we need to remove jcenter() when JCenter is shutdown?

[jjliu15]

I removed jcenter() altogether, since according to https://blog.gradle.org/jcenter-shutdown support will be removed in future versions of gradle.