Fix several concurrency issues

[mvanbeusekom]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fixes

⤵️ What is the current behavior?

There are currently a few related bugs in the geolocator:

• When calling getCurrentPosition after getPositionStream the stream will be cancelled when getCurrentPosition completes;
• When calling getCurrentPosition twice (before the first is finished) the first call gets forgotten and will be ignored (leak);
• In some cases the getPositionStream is not closed correctly which makes it run forever.

🆕 What is the new behavior (if this is a feature change)?

This PR solves the above problems.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

📝 Links to relevant issues/docs

• Fixes Can't stream location after twice getCurrentPosition #546
• Fixes GeolocationManager.java:18: warning: [deprecation] PluginRegistry in io.flutter.plugin.common has been deprecated #556
• Fixes GPS is not turned off after getCurrentPosition() #541
• Fixes [iOS real devices] BackgroundLocationIndicator shows the app is using location service in background forever even when cancelling location stream explicitly when app moves into background #485

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[codecov]

Codecov Report

Merging #563 into develop will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop      #563   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        15    -1     
  Lines          248       214   -34     
=========================================
- Hits           248       214   -34     

Impacted Files	Coverage Δ
...src/implementations/method_channel_geolocator.dart	100.00% <100.00%> (ø)

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