Android guidelines restructuring

[vcolin7]

Fixes issue #482 from the Android repository.

Re-organized the Android guidelines following the @bsiegel's effort for iOS. There are a number of TODO items, but I think it's important to get the ball rolling with a PR to get valuable feedback. These guidelines took heavy inspiration from the iOS guidelines where appropriate, so much of the feedback given by @annelo-msft will be reflected here as well.

[annelo-msft]

Approved, pending resolution of comments.

[annelo-msft]

nit: you took out the TODO without adding the into sentence 😊 (also a couple places below)

[vcolin7]

I noticed the Java guidelines do not have an introductory sentence for these sections and assumed it was not required. Do we still want to add one?

[annelo-msft]

nit:

On the top level nav bar, there are still pages that will need to be consolidated:

This is what it should look like:

[vcolin7]

nit:

On the top level nav bar, there are still pages that will need to be consolidated:

This is what it should look like:

I thought I had already made this change, thanks for pointing it out :)

[vcolin7]

@annelo-msft I did remove the additional links from the general_sidebar.yml file, so I'm not sure why you can see them when generating the HTML. Is there another place I should remove them from?

[annelo-msft]

I was looking at the published site as part of my status check - if you've generated the HTML and they're gone, then we're all good. Apologies for the false alarm.