[camera] Fix a disposed CameraController error thrown when changing a camera

[bselwe]

Fixes a disposed CameraController error thrown when changing the camera in the camera example.

The error is thrown by the ValueListenableBuilder in the CameraPreview. Changing a camera disposes the previous CameraController before the widget state is updated causing the ValueListenableBuilder to listen to the disposed controller. This PR solves the issue by disposing the previous CameraController after the new one is initialized.

Error:

The following assertion was thrown building CameraPreview:
A CameraController was used after being disposed.
Once you have called dispose() on a CameraController, it can no longer be
used.

The relevant error-causing widget was:
  CameraPreview
  file:///Users/.../Projects/plugins/packages/camera/camera/example/lib/main.dart:188:16

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman @BeMacized @felangel

[ditman]

The test failure seems like a chrome flake:

UnknownException (13): unknown error: Chrome failed to start: exited abnormally.
  (unknown error: DevToolsActivePort file doesn't exist)
  (The process started from chrome location /usr/bin/google-chrome is no longer running, so ChromeDriver is assuming that Chrome has crashed.)
  (Driver info: chromedriver=91.0.4472.101 (af52a90bf87030dd1523486a1cd3ae25c5d76c9b-refs/branch-heads/4472@{#1462}),platform=Linux 5.4.104+ x86_64)

I'll restart the test.

[ditman]

Hey Bodhi / Maurits, can you take a look at this? Why wasn't this needed earlier? This was discovered while Bart (@bselwe) developed the Web version of the plugin. Thanks!

[BeMacized]

Why wasn't this needed earlier?

I'm not entirely sure, but I can reproduce the issue and this does look like the appropriate fix to me.

[BeMacized]

LGTM!