ThemeUtils: Retain the original package name in themed UI context

In order for themes to reflect on system elements, we're changing
their context, including the package name.

This breaks anything that tries to identify system elements,
most notably any users of the uiautomator. Trying to do
'UiSelector().packageName("android")' to find, for example,
a force-close dialog will fail.

Fixes com.android.cts.uiautomatortest.CtsUiAutomatorTest#testUiWatcher

Change-Id: Ic05c3024b20ac3772ed5597846fbfa4a184ebb0d