P3A for android

[iefremov]

To launch P3A on android we have to properly inform users about this feature.
Until then, P3A service is disabled in the code for android (see BraveBrowserProcessImpl::brave_p3a_service() usages.

[davidtemkin]

Added @anthonypkeane to drive mobile UX for notification and onboarding for existing and new users -- should parallel desktop, but will have to be custom-designed.

[bsclifton]

Let's use this issue to capture the Android specific work needed to get P3A working on Android. #6688 captures the UI changes needed

[srirambv]

@AlexeyBarabash any reason why the issue is marked as QA/No? Shouldn't we test the changes for P3A? The associated PR has a proper test plan mentioned in it

[AlexeyBarabash]

at that time it required modified apk, which would hard to provide to QA team, and also the task wasn't complete without UI part, so it was marked QA/No

With new changes at #12723 it requires a new test plan