Fix the check on apex-versions for boot images.

This change replaces `kIsTargetBuild` with `kIsTargetAndroid` to avoid
crashes in some environment that has kIsTargetBuild = true but is not
Android (e.g., ARM benchmarking environment).

Bug: 211973309
Test: Presubmits
Change-Id: If9730300ba81d5f82cc187558ba2c5d92811d74b