Get blocked restore keys directly from UserBMS

Bug: 145126096
Test: atest KeyValueRestoreExclusionHostSideTest
      atest PerformUnifiedRestoreHostSideTest
      atest UserBackupPreferencesTest

Currently PerformUnifiedRestoreTask gets the list of blocked restore
keys at construction. However, at that point the list might not be fully
constructed yet. We should get the keys through the getter avaialble in
UserBMS when we need them.

Change-Id: I62ad34138ba7a893e66d6af05d2e242c9c964a44