[res] Optimize idmap format for lookups

Idmap format is currently storing sorted mappings for the
overlays: be it target id -> overlay id, or target id ->
frro data, or the reverse overlay id -> target id list

All of these require binary search for finding the right entry,
and this binary search always touches just 4 bytes of the key,
skipping over all remaining bytes of value. This usually doesn't
make much of a difference for the smaller idmaps but in case of
the larger ones binary search has to load a whole bunch of
RAM into the CPU cache to then throw at least half of it away.

This CL rearranges all mappings into two separate lists: the
first one only contains the sorted keys, and the second one
stores the corresponding data in the same order. This means the
search can only touch the minimum amount of RAM and disk pages,
and then jump exactly to the value of the element it found.

We don't have any benchmarks that would _directly_ capture the
speedup here, and the Java resources_perf ones are too noisy to
make a clear call, but overall they look like some 3-5% speedup
for the overlaid lookups

Test: atest libanrdoidfw_tests idmap2_tests libandroidfw_benchmarks
Flag: EXEMPT performance optimization
Change-Id: I450797f233c9371e70738546a89feaa0e683b333