manifest_fixer: Set targetSdkVersion to '16' for libraries

manifest_fixer.py sets a lib's targetSdkVersion to '15' if it is
not explicitly set.  But ManifestMerger will add dangerous
permissions CALL_LOG_READ/CALL_LOG_WRITE if the following
conditions are met:

    - The app's targetSdkVersion is >16
    - A linked lib's targetSdkVersion is <16
    - The app has CONTACTS_READ/CONTACTS_WRITE permissions

This condition is triggered for some (OEM) app builds (of
Settings).  Update manifest_fixer.py to use '16' as its default
targetSdkVersion so that manifest_merge doesn't add these
dangerous permissions.

Change-Id: Id5f41f7da98c190c8d145fceba0085cf473f4c7a