Fix Heap use after free in clearkey getSecureStops

Security Vulnerability fix:- Heap use after free in getSecureStops
in android.hardware.drm@1.1-service.clearkey

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: 137878930
Change-Id: I78b2dc2bccde238a06398b3733cea8e574ea8ee7