Remove overlay logic from package manager and parser

Consolidate all knowledge about overlays to the overlay manager (except
static RROs that target "android": these are handled from native code in
order to be loaded as part of Zygote boot).

This removes the ability to overlay AndroidManifest.xml. There are some
use-cases where overlaying a manifest makes sense [e.g. changing an
app's label or icon or what components should be enabled by default] but
the opposite is also true [e.g. permissions]. Support for manifests can
be introduced again gradually with these goals in mind:

  - should not happen during parsing [which represents the immutable
    truth as read from disk]

  - should happen during scanning [which applies mutable settings to a
    parsed package]

  - be limited in what can be modified

Bug: 78808368
Test: builds, boots
Change-Id: I12c16fce65b1e68876f0c9acd7e2a61405e64435