Fixing a ClassCast exception with ErrorFragments resulting

from them expecting to only be called by
UninstallerActivity.

Test: Covered by a gts-test ag/1806256 and manually tested
by calling from com.google.vending (Play Store), available
at http://sponge/581c3cc1-534c-4d0e-834f-0ba4cf17ac68.

Fixes: 35430100
Change-Id: Ic5d0e3832163b2e0599fc264361ed49c9333675d