Use scheduled writes to package-restrictions and user xml

When user is initialized, PackageManager.setEnabledSetting is called
multiple times. Each call triggers a write to user's
package-restrictions.xml file.

Using scheduleWritePackageRestrictionsLocked should help to reduce the
number of writes during initialization time and reduce creation/switching
time for a new user.

Change-Id: I5bd8609e3a79bdc3f198a169ede8005dc0186612