provideKeyResponse to send an event to test AMediaDrm_setOnEventListener.

The drm plugin provides a listener, we send a kDrmPluginEventVendorDefined
if provideKeyResponse succeeds.

The reason for sending kDrmPluginEventVendorDefined is because
kDrmPluginEventKeysChange currently does not have ndk api support.

This addition is intended for testing AMediaDrm_setOnEventListener only.

Test: cherrypick go/ag/1598039 and go/ag/1630324

ANDROID_BUILD_TOP= ./android-cts/tools/cts-tradefed run cts -m CtsMediaTestCases --test android.media.cts.NativeClearKeySystemTest#testClearKeyPlaybackCenc

Test passes and look for "EVENT_VENDOR_DEFINED received" in the debug log.

bug: 32648208
Change-Id: Iec98b856e8d4ce888f8a1cd2568a3dee871effc5