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: I63c1b8b7622a6f45cf7555e5b8bc26de6b6925cc