docs: Adding P to the API levels

Based this on SMain@ 's changes in ag/1936024 , which added the O
API level for the O preview.

Doc is staged to:

go/dac-stage/reference/

For an example of a new API class with the watermark, see:

go/dac-stage/reference/android/media/MediaPlayer2.html

Test: make ds-docs
Bug: 73009741
Change-Id: I591419c903222c9ad0c541dcd4bfce6a38ba1f2c