[res] Remove unused fields from ResolvedBag::Entry

The pointers to the key and type string pools were never used,
but they take 20 bytes for each object, doubling its size.

This change reduces the struct size 40 -> 20 bytes, so all
style and array resolution code should get a bit faster

Bug: none
Test: atest libandroidfw_tests
Flag: EXEMPT minor cleanup
Change-Id: I168df90af241a3edd5779b9ef609893b1be1829c