Update the version of Kotlin and Gradle

[maeda-kazuya]

Hi, I updated app/build.gradle and gradle-wrapper.properties by the latest Kotlin/Gradle version.
Somehow I couldn't resolve the dependency of 'android-P', so I also replaced it with 28.

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[maeda-kazuya]

I signed it!

[googlebot]

CLAs look good, thanks!

[dazza5000]

instead of hardcoding this here, why not update the gradlePluginVersion variable?

[maeda-kazuya]

Thanks for checking, I fixed it. Kotlin ver1.3.10 was released, so I also updated Kotlin to the version.

[dazza5000]

A few merge conflicts today

[maeda-kazuya]

@dazza5000 Thanks, I fixed merge conflicts, so please check it out.