Change recording format to M4A

[ChaoticNeutralCzech]

Issue description

This app stores recordings in the MP4 container, which is perfectly fine for audio. However, the extension is .mp4 and the current version of VLC and built-in video for Android, as well as various other apps that open, process or send media files, expect these files to contain a video track, and its absence makes them treat the file as invalid.

Therefore, I propose a very minor change: the file extension should be .m4a (MPEG-4 Audio). The internal structure of .m4a files is identical to normal MP4's, the only difference is that the extension makes applications expect an audio-only file. This has been supported for many, many years.

Proposed solution

The code contains 5 instances of mp4 in total. Most of them (🟢) can just be replaced with m4a. Listed in order of importance:

1. /app/src/main/java/com/danielkim/soundrecorder/RecordingService.java : 115
   • 🟢 File extension of newly created recording.
2. /app/src/main/java/com/danielkim/soundrecorder/adapters/FileViewerAdapter.java : 256
   • 🟢 Extension appended to text input from rename dialog
3. /app/src/main/res/layout/dialog_rename_file.xml : 15
   • 🟢 Rename dialog: text that appears right from textbox
4. /app/src/main/java/com/danielkim/soundrecorder/adapters/FileViewerAdapter.java : 237
   • ⚠️ MIME type of recorded audio when starting player intent, currently audio/mp4. It could work if replaced with audio/x-m4a or audio/m4a; however, I think keeping it as-is is best for compatibility.
5. /app/src/main/res/layout/fragment_media_playback.xml : 33
   • 🟢 Placeholder text, replaced before being shown on screen. Replacing with m4a recommended but not neccessary.