[Android] Using a well-formed IETF language tags instead of debug string

[proninyaroslav]

Changes description

This PR replaces all Locale#toString() with Locale#toLanguageTag() for Android 5.0 and above.

Reason for changes

The plugin returns a debug string instead of a locale tag. For example, instead of zh_Hant_TW (lang_script_country) we got zh_TW_#Hant (lang_country_#script). This is contrary to the Dart locale tag format. Dart and intl package uses IETF BCP 47 format and IANA Language Subtag Registry to determine locale: https://api.flutter.dev/flutter/dart-ui/Locale-class.html.
When passing a string like zh_TW_#Hant instead of zh_Hant_TW to the intl.Locale.parse we got a FormatException.

The documentation for Android Locale#toString() says:

Returns a string representation of this Locale object, consisting of language, country, variant, script, and extensions as below:
language + "_" + country + "_" + (variant + "_#" | "#") + script + "-" + extensions

This behavior is designed to support debugging and to be compatible with previous uses of toString that expected language, country, and variant fields only. To represent a Locale as a String for interchange purposes, use toLanguageTag().

instead of it, the Locale#toLanguageTag() says:

Returns a well-formed IETF BCP 47 language tag representing this locale.

[proninyaroslav]

@magnatronus
Can you take a look at this?

[proninyaroslav]

@magnatronus
Please merge this PR

[magnatronus]

Hi, I am not in a position to check and do the PR currently. As soon as I am I will take a look.