Make back-facing camera optional in manifest

Merged akwizgran requested to merge back-facing-camera-not-required into master

After making touchscreen support optional, the Google Play Console shows that Briar supports some Chromebooks. But most Chromebooks are still shown as unsupported. The device catalogue says "This device model is not supported in your app's APK manifest and hence users of this device model cannot install your app", but doesn't give any more information.

My Chromebook (Asus C202SA, Android 7.1) isn't supported, so I can't install Briar from Google Play. But it works when I install it via ADB.

As far as I can tell, the supported devices have the hardware feature, which indicates a back-facing camera, whereas the unsupported devices have and We support front-facing cameras, so we should make optional.

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