Remove IntDef annotations from NetworkPolicyManager

@RestrictBackgroundStatus and @MultipathPreference are defined
in connectivity mainline module. Annotate these in non-updatable
platform is not going to be manageable long term. For example,
if the module gets updated and the annotation gets more values
then it's a problem in non-updatable platform. So, it should
not have IntDef cross module boundaries. The javadoc already
document the return type.

Bug: 183972925
Test: build
Change-Id: I371e61fc1ba4d6137139bcfbf1d57fdebe00a257