Fixed AMediaDrm_getKeyRequest returning both new and old requests.

If multiple key requests are made through AMediaDrm_getKeyRequest,
(e.g. Service Certification + key request in private mode), the
new request will be appended to the old request/s.

AMediaDrm_getKeyRequest should just return the most recent request.

Test: CTS test
  Modify testQueryKeyStatus to make two back to back queries. The test
  should pass.

Test: CTS test
  ANDROID_BUILD_TOP= ./android-cts/toolsefed run cts-dev --module CtsMediaTestCases
  -t android.media.cts.NativeMediaDrmClearkeyTest#testQueryKeyStatus

bug: 77713912
Change-Id: I21a30572fedc67cdf1d813ea36b92d2a67b4ebb4