Switch from empty AndroidManifest to namespace

[romtsn]

Description

AGP will not allow using package attribute on an empty AndroidManifest starting from version 8.0 (we use it a lot, since we publish android libraries without UI). We should specify a namespace attribute in the android { } Gradle DSL instead.

Some automation script on how to do it: https://twitter.com/JakeWharton/status/1540075477108490246?s=20&t=VN5FPQPh_Nf87VXw6038ng

This will also avoid the manifest merging task altogether

[philipphofmann]

@romtsn, can you maybe add the most important info for this issue here and mention the link as a resource? The tweet could disappear and then the info is lost.

[stefanosiano]

merged into master