Keystore: Fix KeyGenParameterSpec parceling

Fix the way KeyGenParameterSpec is parceled, by correctly handling
default and null values for some of the fields.

A recent CL added the ability to parcel/unparcel KeyGenParameterSpec (by
a separate class).
Due to refactoring late in the CL review cycle, the parceling code did
not take into account a few edge cases.

Unit tests:
m -j KeystoreTests && adb install -r out/target/product/marlin/data/app/KeystoreTests/KeystoreTests.apk
adb shell am instrument 'android.security.tests/android.support.test.runner.AndroidJUnitRunner'

CTS tests:
cts-tradefed run commandAndExit cts-dev -a armeabi-v7a -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testKeyManagement -l DEBUG

Bug: 69337278
Test: Keystore unit tets (see instructions above) and cts Key Management test.
Change-Id: Ie08f42b07fb55b6fa1d8fb73c89d69687c97e214