Validate decryption key length to decrypt function.

Cherry picked from http://go/ag/3038278.

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.

Add DecryptsWithEmptyKey and DecryptsWithKeyTooLong unit tests.

Test: ClearKeyDrmUnitTest
  adb shell LD_LIBRARY_PATH="/vendor/lib/mediadrm"
  /data/nativetest/ClearKeyDrmUnitTest/ClearKeyDrmUnitTest

bug: 63982768
Change-Id: I1f22c9df2b051972b2c532608b7f203e3ce77926