[res] Use fstat() for idmap::IsUpToDate()

The most common operation when getting a new Resources object
is checking if all apks and overlays are still up to date to
reuse the cached object. It makes sense to optimize it by
excluding the file by path lookups and instead keeping an open
FD to the file in the cache

+ Make IsFabricatedOverlay() more efficient via a name check
  and string_view where possible

Bug: 282215580
Test: build + boot + presubmit
Change-Id: Ib1ab20cba359c2195a72dd2e10096883d95b4453