Let AndroidComponentAddress specify a target UserHandle

[jdcormie]

The target UserHandle is best modeled as part of the SocketAddress since it's part of the server's location.

This change allows a NameResolver to select different target users over time within a single Channel.

[ejona86]

I know this is draft, but a few drive-by comments. Looks fair.

[wwtbuaa01]

@ejona86 This change LGTM.

Discussed with John offline for this change.

Making the target UserHandle as part of the AndroidComponentAddress makes sense for two reasons:

1. More and more OEM privileged applications (Google 1P applications internally) are onboarding with X-user usages, and all of them are only specifying target userHandle with INTERACT_ACROSS_USERS privilege permission pre-granted.
2. Currently, optimization for channel creation is done in Google internal codebase. After this change, NameResolver + AndroidComponentAddress.getTargetUser can be utilized by other codebase, such as Android system image.

[jdcormie]

Thanks Wentao!