feat(android): add option for showing internal/external storage

[xuhcc]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• All existing tests are passing
• Tests for the changes are included

What is the current behavior?

Internal and external storage may not be shown by default in Android file chooser.

What is the new behavior?

The new showAdvanced option forces the file chooser to show these locations. However, under the hood, it uses an undocumented android function, so the warning was added to readme.

I also removed two broken links from TOC.

[tgpetrov]

@xuhcc We are experiencing an issue with saucelabs where we run our tests which is the reason for their failure, as soon as we sort it out we can proceed with merging your PRs.

[tgpetrov]

@xuhcc Can you rebase your branch with latest changes from master so a can merge it?

[xuhcc]

@tgpetrov Done.