[generator] Don't mark a method as [UnsupportedOSPlatform] if it overrides a supported base method.

[jpobst]

Context: #1307
Context:

MAUI noticed this API says UnsupportedOSPlatform:
// Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065
// Android.OS.Bundle
[UnsupportedOSPlatform("android21.0")]
[Register("putString", "(Ljava/lang/String;Ljava/lang/String;)V", "")]
public override void PutString(string? key, string? value) { ... }

While technically correct that android.os.Bundle.putString (...) was "removed" in API-21, it was really moved to a base class as android.os.BaseBundle.putString (...). We will continue to generate both methods for binary compatibility, but we should not mark the override method as [UnsupportedOSPlatform] because the "supported" base method is still present.

Fix generator to recognize this case and not emit [UnsupportedOSPlatform] here.

With this change, Android.OS.Bundle.cs contains:

// Metadata.xml XPath method reference: path="/api/package[@name='android.os']/class[@name='Bundle']/method[@name='putString' and count(parameter)=2 and parameter[1][@type='java.lang.String'] and parameter[2][@type='java.lang.String']]"
[Register ("putString", "(Ljava/lang/String;Ljava/lang/String;)V", "")]
public override unsafe void PutString (string? key, string? value) { ... }