Identity.SignOut: Apply code convention on instance + synchronized + post background

[guperrot]

Change warning to error message for cannot sign out and update wording.

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Description

Missed feedbacks last time on sign out.

[guperrot]

Variables always accessed in background thread do not require extra synchronization.

[MatkovIvan]

@guperrot As I said in the comment to that PR, mSignInDelayed variable can be accessed from the UI thread too.

https://github.com/Microsoft/AppCenter-SDK-Android/blob/12a347d6848aa4a2edf236bc0c796e113e3697e6/sdk/appcenter-identity/src/main/java/com/microsoft/appcenter/identity/Identity.java#L233

[guperrot]

@guperrot As I said in the comment to that PR, mSignInDelayed variable can be accessed from the UI thread too.

AppCenter-SDK-Android/sdk/appcenter-identity/src/main/java/com/microsoft/appcenter/identity/Identity.java

Line 233 in 12a347d

if (mSignInDelayed) {

We are getting rid of that with the new bug anyway but can be fixed simply via #984 while keeping the pattern of using post inside the instance method.