Add KEY_TYPE_OFFLINE support.

Clearkey now supports KeyType Streaming and Offline.

Test: CTS test
  android.media.cts.MediaDrmClearkeyTest#testClearKeyPlaybackOfflineCenc
  android.media.cts.MediaDrmClearkeyTest

Test: VTS tests
  adb shell /data/nativetest/VtsHalDrmV1_0TargetTest/VtsHalDrmV1_0TargetTest
  adb shell /data/nativetest/VtsHalDrmV1_1TargetTest/VtsHalDrmV1_1TargetTest

bug: 38112718
Change-Id: I34c788cd9457c6756de8bca1b3b22b9f1ff0655a