Fix mozilla#177: Disable price polling in Private Browsing windows

Update `shouldUpdatePrices` with a check for whether or not the active window is a private window.

I did not add a check for `isActiveWindowPrivate` in `shouldExtract` in `./src/privacy.js`, despite the fact that `browser.extension.inIncognitoContext` returns the "wrong" value for background extraction, because background extraction is gated by the `shouldUpdatePrices` check.

src/privacy.js

@@ -44,8 +44,7 @@ export async function shouldCollectTelemetry(method) {
  * @return {boolean}
  */
 export async function shouldUpdatePrices() {
-  // TODO (bdanforth): Add private browsing check per #177
-  return true;
+  return !isActiveWindowPrivate();
 }
 
 async function trackingProtectionEnabled() {
@@ -69,3 +68,8 @@ async function cookiesBlocked() {
 
   return true;
 }
+
+async function isActiveWindowPrivate() {
+  const activeWindow = await browser.windows.getCurrent();
+  return activeWindow.incognito;
+}