[res] Optimize name lookups in idmap service

The most common operation when build idmaps is to find the
resource ID by name. This is the least optimized operation
in the current resources data structures, as they only
expect ID -> value lookups

This change adds an optional flag that creates a name->ID
hash map inside ResStringPool, and use it when loading
APKs in idmap2d

Bug: 282215580
Test: build + boot + perf record

Change-Id: I82f4d684cb48e2dcddcd677b882b11497c1c13b1