playback: remove deprecated setFlags

[hades]

Per [1] these flags have been deprecated in API 26.

Tested with an API level 21 emulator: playback controls still work as expected.

[1] https://developer.android.com/reference/android/media/session/MediaSession#FLAG_HANDLES_MEDIA_BUTTONS

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[ByteHamster]

I would just keep the flags for now. When we rewrite the service we will probably remove them then. I don't think it is worth risking to break something (eg on older Android versions) just as a code cleanup. If you are interested in working more on the playback service, we can talk in more detail on the community call if you want. For example, we could restructure parts of the sleep timer as preparation for the new playback service