Prepare for switch to idmap2

Prepare the idmap and asset managers for interfacing with idmap2 instead
of today's installd + idmap pipeline, but don't make the switch just
yet.

Instead, idmap2 runs as its own native daemon with an AIDL interface.
This removes the need for installd to fork and exec on each idmap call,
saving about 50 ms per call.

Bug: 78815803
Test: atest OverlayDeviceTests OverlayHostTests
Change-Id: I60852e15d99329896ff9de6559d1e7cd1c67e33d