[geolocator_android] Introduce option to force a specific location provider Issue/1171

[jbxbergdev]

Added an option AndroidSettings.forceProvider that forces the plugin to use a specific provider out of the currently four available Android location providers. As discussed with @mvanbeusekom, this option is to be used in conjunction with forceLocationManager = true.

Example use case: An app needs to ensure locations are derived exclusively from GNSS and hence has to make sure no other sources like mobile networks are used.

#1171

Pre-launch Checklist

• [x] I made sure the project builds.
• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
• [ ] I updated CHANGELOG.md to add a description of the change.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I rebased onto main.
• [x] I added new tests to check the change I am making, or this PR does not need tests.
• [x] I made sure all existing and new tests are passing.
• [x] I ran dart format . and committed any changes.
• [ ] I ran flutter analyze and fixed any errors.