Add tests to validate key length for clearkey plugin.

AesCtrDecryptor::decrypt() doesn't check whether the size of "key"  is
equal to 16 bytes,  which may lead to an OOB read problem in the context
of mediadrmserver. The fix is in clearkey plugin. Add tests to validate
the fix.

Test: VTS test
  adb shell /data/nativetest/VtsHalDrmV1_0TargetTest/VtsHalDrmV1_0TargetTest

bug: 63982768

Merged-In: Ife2da17e7f39d8031bc36b83c3b27ba5e9d83eb7
Change-Id: Ife2da17e7f39d8031bc36b83c3b27ba5e9d83eb7