WebView Tracing API: address comments from the API council.

Address comments from the API council regarding the WebView Tracing API.

android.webkit.TracingController:
 * start() throws IllegalStateException if the system is already tracing
   (this is instead of returning false)
 * stop uses Executor instead of Handler
 * removed stop() (the method w/o arguments)
 * renamed stopAndFlush to stop
 * use OutputStream instead of a custom callback interface TracingOutputStream
 * dropped requirement for UI threading
 * updated documentation

android.webkit.TracingFileOutputStream:
 * removed the TracingFileOutputStream file completely (functionality
   replaced by the existing FileOutputStream)

android.webkit.TracingConfig:
 * removed example with CATEGORIES_NONE and “-input,-gpu”.
 * customCategories are List<String> instead of String
 * updated documentation
 * added two more predefined categories: CATEGORIES_ALL,
   CATEGORIES_ANDROID_WEBVIEW
 * some refactoring, added a Builder class
 * ensure that only include category patterns can be specified
 * uniform addCategories interface for construction
 * predefined category sets are a bitmask now

BUG: 71584598,71584599,63750258
Test: CTS
Change-Id: I615ef5f43d26968329182b09e7c26178f1f85ecc