platform_testing: Add external documentation for platform testing.

Adapts much of go/apct-guide for general android development,
focusing on AOSP and the new test configurations that will be
released with O.

Scrubbed references to internal code-search and go links.

Bug: None
Test: Markdown Preview
Change-Id: Id38c78f7b7366449d5cfb52939e79e6e364d4728