(android) Don't overwrite the whole SystemUiVisibility in setStatusBarTransparent

[mohammadrafigh]

Fixes #275

Platforms affected

Android

Motivation and Context

in setStatusBarTransparent function, current SystemUiVisibility is being replaced by the new one, which is incorrect as stated in #275.

Description

Like other places in the same codebase, I just called getSystemUiVisibility before applying the new flags and used current visibility as a base option for those flags.

Testing

It's already mentioned in #275.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary