Store "block uninstall" flag separately from the rest of package state.

This allows to set "block uninstall" prior to installation and avoid the
inevitable race that happens when Device Policy app tries to force install and
then immediately block uninstall.

BUG=31043188
Test: Block com.chrome.beta in TestDPC. install, fail to uninstall through adb, unblock, uninstall

Change-Id: I5ffa2abcb003982eccfb77585c43b59532dd501d