Minor refactor after merging #52.

Add support for hiding the Toolbar on Android

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Add support for bottom tabs in Android

Removed propTypes from example-redux screens

Adds BottomTabActivity which replaces TabActivity
Adds a dependency on `com.aurelhubert:ahbottomnavigation:1.2.3`
Uses iOS tabsStyle, with additional parameter that allows toggling of whether to show the titles of inactive tabs

Add support for passing props to a screen in Android

Anything passed in a navigator event in a 'passProps' object will be serialized into the Java side Screen object as a HashMap<String, Object>. When RctView starts the React root the props are deserialized into the Bundle passed as props, along with the navigation info. Due to React and Java limitations, JS arrays of arrays are deserialized as dictionaries by index, and arrays of multiple types are not supported.

Also changed the structure used to store Modals
from a Map based on navigatorId to a stack.
Reason for the change is that screens can be
pushed only to the topmost Modal.