Fix theme provider crashing for non-primary user.

PackageManagerService's context is for user 0, so if the theme provider
for other users was invoking the getInstalledThemePackages() method, the
themes provider crashed as it doesn't have the INTERACT_ACROSS_USERS
permission.
Fix that (and simplify code) by making getInstalledThemePackages()
return the installed themes for the current user, which was probably the
intent anyway.

Change-Id: I02002cee005b74128f3b2f7cc7a93b9b8514f1f3
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
Theme fixups

Change-Id: Ic2c027b1815e5ccb85c660b7e123df1469d903a2