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

Android.mk

@@ -195,6 +195,7 @@ framework_docs_LOCAL_DROIDDOC_OPTIONS := \
     -since $(SRC_API_DIR)/25.txt 25 \
     -since $(SRC_API_DIR)/26.txt 26 \
     -since $(SRC_API_DIR)/27.txt 27 \
+    -since ./frameworks/base/api/current.txt P \
     -werror -lerror -hide 111 -hide 113 -hide 125 -hide 126 -hide 127 -hide 128 \
     -overview $(LOCAL_PATH)/core/java/overview.html \