Add first-class support to ReactInstanceManager for Activity-less usage

Summary:
I'm not sure if we'll ever need to do more than this (probably!) but I want to have a specific option for Activity-less usage, so
that our logic in ReactInstanceManager and elsewhere is more clear. I want to be able to confidently assert that the Activity is present
and correct, or never present, depending on if this `requireActivity` flag is set; instead of adding else statements that imply "well, the Activity should be here, hope everything is okay" which feels hacky.

Doesn't change much for now, but it's an additional constraint in the RN Android codebase that we need to explicitly embed in code so that
we can point to it, and so that the logic is more clear.

Changelog: [Internal]

Reviewed By: javache, motiz88

Differential Revision: D30504616

fbshipit-source-id: d2abdb7c4765a16113c9517406cdbb8cf42822ff

ReactAndroid/src/main/java/com/facebook/react/ReactNativeHost.java

@@ -68,6 +68,7 @@ public abstract class ReactNativeHost {
             .setApplication(mApplication)
             .setJSMainModulePath(getJSMainModuleName())
             .setUseDeveloperSupport(getUseDeveloperSupport())
+            .setRequireActivity(getShouldRequireActivity())
             .setRedBoxHandler(getRedBoxHandler())
             .setJavaScriptExecutorFactory(getJavaScriptExecutorFactory())
             .setUIImplementationProvider(getUIImplementationProvider())
@@ -124,6 +125,11 @@ public abstract class ReactNativeHost {
     return null;
   }
 
+  /** Returns whether or not to treat it as normal if Activity is null. */
+  public boolean getShouldRequireActivity() {
+    return true;
+  }
+
   /**
    * Returns the name of the main module. Determines the URL used to fetch the JS bundle from Metro.
    * It is only used when dev support is enabled. This is the first file to be executed once the