Add a new API to import a key provided by the caller, such that this key

can also be synced to the remote service

This API may be useful for backward-compatibility work, e.g., recovering
a key that's backed up in Android Q+ to Android P without updating the
Android P Frameworks code. This API may also be useful for other use cases.

Bug: 73785182
Change-Id: I1022dffb6a12bdf3df2022db5739169fcc9347d2
Test: adb shell am instrument -w -e package \
com.android.server.locksettings.recoverablekeystore \
com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner