Remove the reliance of callbackUriHost and callbackUriScheme when those values are not using.

[Achintha444]

Description:
Currently in the Android SDK, we need to add callbackUriHost and callbackUriScheme to setup even those values are not using.

ex:

android.defaultConfig.manifestPlaceholders = [
    'appAuthRedirectScheme': 'wso2sample',
    'callbackUriHost': '',
    'callbackUriScheme': ''
]

This can be an overhead for the developer. Therefore we need to fix this by removing the necessity on adding the callbackUriHost and callbackUriScheme when those values are not using.