Clear binder identity before querying package information

DevicePolicyManagerService needs to clear caller identity before
calling into PackageManager APIs, to make sure the app enumeration
restriction in R does not adversely affect its functionalities.

Bug: 150407679
Test: MixedManagedProfileOwnerTest#testDelegatedCertInstaller
      (without the stopgap fix ag/10456865)
Change-Id: I237c527241c26a309302bc2f7e36f8007a6c53b8