Add Android support to add image as background overlay

[meierrap]

Adding Android support for #36

Add support for images on RNPreventScreenshot.enableSecureView method as argument.
Supports all types of URI such as https:// or file://

Solves #34

[killserver]

pls add support for ios(maybe empty "fallback") )

[meierrap]

Ios support is already done in this pr #36, I just fixed android side

[RVassili]

Hey @killserver , the iOS méthod already supports the background image, see

react-native-screenshot-prevent/ios/RNScreenshotPrevent.m

Lines 185 to 192 in 3c5302e

	RCT_EXPORT_METHOD(enableSecureView: (NSString *)imagePath) {
	if(secureField.secureTextEntry == false) {
	UIView *view = [UIApplication sharedApplication].keyWindow.rootViewController.view;
	for(UIView *subview in view.subviews) {
	[self addSecureTextFieldToView:subview :imagePath];
	}
	}
	}

[killserver]

Ios support is already done in this pr #36, I just fixed android side

thx!)