Docs: Add demos and testapps documentation.

* Add new README files for the Transformer demo and media/testapps/.
* Add an androidx demos README in media/github/

PiperOrigin-RevId: 396784430

google3/third_party/java_src/android_libs/media/github/README-androidx-demos.md

+# Android media demos #
+
+This directory contains applications that demonstrate how to use Android media
+projects, like ExoPlayer.
+Browse the individual demos and their READMEs to learn more.
+
+## Running a demo ##
+
+### From Android Studio ###
+
+* File -> New -> Import Project -> Specify the root `media` folder.
+* Choose the demo from the run configuration dropdown list.
+* Click Run.
+
+### Using gradle from the command line: ###
+
+* Open a Terminal window at the root `media` folder.
+* Run `./gradlew projects` to show all projects. Demo projects start with `demo`.
+* Run `./gradlew :<demo name>:tasks` to view the list of available tasks for
+the demo project. Choose an install option from the `Install tasks` section.
+* Run `./gradlew :<demo name>:<install task>`.
+
+**Example**:
+
+`./gradlew :demo:installNoExtensionsDebug` installs the main ExoPlayer demo app
+ in debug mode with no extensions.