Harden Clearkey releaseSecureStops implementation.

Perform more checking for invalid input.

Test: CTS tests
  android.media.cts.MediaDrmClearkeyTest#testSecureStop
  android.media.cts.MediaDrmClearkeyTest

Test: run drmpoc
  no signal 6 on clearkey service after the fix

Test: adb shell ps | grep clearkey
  pid ID does not change after running drmpoc

bug: 137284652
Change-Id: I971bb33eec6d37ef86fa1a53501c1e6bda50fa3b