Check themes of current user, not user 0.

Change-Id: I5d685a04289b434a12e460b4fdacc43a41ff2be1
JIRA:CYAN-1303
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
Allow theming of lockscreen

Change-Id: I10799482e0b393746b4c9818f94862109fac6006
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