Fixed DPMS.listForegroundAffiliatedUsers().

It was not properly clearing the caller's identity, so it would
fail "in real life" when the DPC doesn't have INTERACT_ACROSS_USERS
(but it was passing on CTS because the test app had it).

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testListForegroundAffiliatedUsers_onlyForegroundUserCalledByDeviceOwner # on automotive
Fixes: 186455856

Change-Id: I2924f5cdf90e767c4f2ae534788baa5c448126c5