SDK / exoplayer

Without this permission, files in `/sdcard` or other directories cannot be read by the demo, as they're not in the transformer demo's scoped storage container.

For more information, see https://developer.android.com/training/data-storage/use-cases

Tested by uninstalling and re-installing the demo app, granting permission by
starting a transformation, and launching an intent using a local file in `/sdcard`
to start another transformation. Without this CL, this threw an error, and with
this CL it succeeded.

PiperOrigin-RevId: 466399023
(cherry picked from commit ec6ed0d5)