Add `true` comparison to jinja statement

Add a missing `true` when comparing a jinja variable. Without this, the
statement always resolves to true causing build failures in multiple
platforms.

TEST=None
BUG=None

Change-Id: Ic5973e7f65c7f7cbef4bc77df6c1d214bec6ddb4
Reviewed-on: https://chromium-review.googlesource.com/775605Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Luke Halliwell <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517407}

chromecast/browser/android/apk/AndroidManifest.xml.jinja2

@@ -26,7 +26,7 @@
                   android:taskAffinity=".CastWebContentsActivity"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize"
                   android:excludeFromRecents="true"
-                  {% if cast_build_enable_background_activities %}
+                  {% if cast_build_enable_background_activities == "true" %}
                   android:noHistory="false">
                   {% else %}
                   android:noHistory="true">