Incorporate API feedback

This change addresses the feedback received from the api council. It
addresses the following:
     * Adds more documentation around widgetCategory
     * Adds the WIDGET_CATEGORY prefix to the StringDefs
     * Add more documentation around RelativeFrameTimeHistrogram and
       why there are negative buckets.
     * adds a MAX bucket to the histogram to mirror the MIN.

Bug: 382041175
Test: Atest CoreAppJankTestCases
Flag: android.app.jank.detailed_app_jank_metrics_api

Change-Id: Ic4e77e08b06fdf03982a5aa1f2b6363d16869205