When sending a custom command with `browser.sendCustomCommand` when
connected to a legacy browser service, the custom command was delivered to `MediaSessionCompat.Callback.onCustomAction` instead of the service method
`onCustomAction`. The difference is that the service version can return an
async response with a bundle, while the session callback version doesn't
have a return value.
Hence, the service method was never called and it wasn't possible to send
a reponse or signal an error back to the browser. The resulting
`ListanableFuture` simply always immediately resolved to a success.
This change overrides `ListenableFuture<SessionResult> sendCustomCommand(SessionCommand command, Bundle args)` in
`MediaBrowserImplLegacy` to use the `MediaBrowserCompat` method to send
instead of the `MediaControlleCompat` method that was used by the subclass
`MediaControllerImplLegacy`. This involves the service callback instead of the
session callback and enables `MediaBrowser` to get the actual return value
from the legacy service.
Issue: androidx/media#1474
#cherrypick
PiperOrigin-RevId: 676519314
6bda0da6be