idmap: optimize time to create idmap data

Change idmap to iterate over the resources in the overlay package
instead of the target package when scanning for resources defined in
both packages. This cuts down the runtime cost of creating an idmap
considerably since the algorithm now scales with the number of resources
in the overlay package (a handful) and not the number of resources in
the target package (android: 10k, SystemUI: 8k) at a minor cost to code
complexity.

Improvements on the runtime of ResTable::createIdmap (systrace on an
emulator running aosp_x86_64-eng):

  - target=android: 12.5 ms -> 3.0 ms
  - target=SystemUI: 8.6 ms -> 1.0 ms

The bulk of the cost of creating an idmap from installd is now the fork
and execl to call "idmap --fd ..." which weigh in at 16 ms.

Bug: 80150169
Test: make libandroidfw_tests
Test: atest OverlayHostTests OverlayDeviceTests
Change-Id: I98e18d5958c0cd835a73055b714f5bf0f4f95a09