Read manifest values using resource id in idmap2

Idmap2 currently reads android:targetPackage and android:targetName
from overlay manifests by looking for an attribute with the name of
the attribute resource.

This fixes the divergence from package parsing by finding the
attributes using their resource ids.

Bug: 175060836
Test: libidmap2_tests
Change-Id: I09965e5880a1e6c48c3f8077db0c595484804ce7