Fail KeySyncTask if the PlatformKey can't be used to decrypt

Currently just uploads an empty set of keys, and thinks the key sync
was successful. (i.e., does not sync again afterwards.)

Bug: 72299798
Test: adb shell am instrument -w -e package
com.android.server.locksettings.recoverablekeystore
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner

Change-Id: I7342f93bd205777688a737d6e1ec8c1ab5b5f7a5