Make README structure match the desired androidx structure

PiperOrigin-RevId: 397319051

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.