Fix clearkey v1.4 issues

Incorrect use of V1_2::ICryptoPlugin, which causes
castFrom to return null pointer in b/186705731.

Fix testGetLogMessages "invalid log priority" exception
in b/186677289. The artificial log message returned in
getLogMessages uses the log message protobuf's DEFAULT
priority, which is not a valid android log priority.

Add getLogMessages for CryptoPlugin and addresses
review comment in ag/13963883.

Test: cts
  atest CtsMediaTestCases:android.media.cts.MediaDrmTest

Test: vts-tradefed
  run vts -m VtsHalDrmV1_4TargetTest
  run vts --include-filter VtsHalDrmV1_[0-4]TargetTest

Bug: 186705731
Bug: 186677289
Bug: 182525516
Bug: 184723035

Change-Id: I6ad3771a3fc622551b74f4f5788e2e40ff87929b