Runtime resource overlay, iteration 2 

Support any number of overlay packages. Support any target package.

UPDATED PACKAGE MATCHING
------------------------
In Runtime resource overlay, iteration 1, only a single overlay package
was considered. Package matching was based on file paths:
/vendor/overlay/system/framework-res.apk corresponded to
/system/framework-res.apk. Introduce a more flexible matching scheme
where any package is an overlay package if its manifest includes

    <overlay targetPackage="com.target.package"/>

For security reasons, an overlay package must fulfill certain criteria
to take effect: see below.

THE IDMAP TOOL AND IDMAP FILES
------------------------------
Idmap files are created by the 'idmap' binary; idmap files must be
present when loading packages. For the Android system, Zygote calls
'idmap' as part of the resource pre-loading. For application packages,
'idmap' is invoked via 'installd' during package installation (similar
to 'dexopt').

UPDATED FLOW
------------
The following is an outline of the start-up sequences for the Android
system and Android apps. Steps marked with '+' are introduced by this
commit.

Zygote initialization
   Initial AssetManager object created
+    idmap --scan creates idmaps for overlays targeting 'android', \
           stores list of overlays in /data/resource-cache/overlays.list
   AssetManager caches framework-res.apk
+  AssetManager caches overlay packages listed in overlays.list

Android boot
   New AssetManager's ResTable acquired
     AssetManager re-uses cached framework-res.apk
+    AssetManager re-uses cached 'android' overlays (if any)

App boot
   ActivityThread prepares AssetManager to load app.apk
+  ActivityThread prepares AssetManager to load app overlays (if any)
   New AssetManager's ResTable acquired as per Android boot

SECURITY
--------
Overlay packages are required to be pre-loaded (in /vendor/overlay).
These packages are trusted by definition. A future iteration of runtime
resource overlay may add support for downloaded overlays, which would
likely require target and overlay signatures match for the overlay to
be trusted.

LOOKUP PRIORITY
---------------
During resource lookup, packages are sequentially queried to provide a
best match, given the constraints of the current configuration. If any
package provide a better match than what has been found so far, it
replaces the previous match. The target package is always queried last.

When loading a package with more than one overlay, the order in which
the overlays are added become significant if several packages overlay
the same resource.

Had downloaded overlays been supported, the install time could have been
used to determine the load order. Regardless, for pre-installed
overlays, the install time is randomly determined by the order in which
the Package Manager locates the packages during initial boot. To support
a well-defined order, pre-installed overlay packages are expected to
define an additional 'priority' attribute in their <overlay> tags:

    <overlay targetPackage="com.target.package" priority="1234"/>

Pre-installed overlays are loaded in order of their priority attributes,
sorted in ascending order.

Assigning the same priority to several overlays targeting the same base
package leads to undefined behaviour. It is the responsibility of the
vendor to avoid this.

The following example shows the ResTable and PackageGroups after loading
an application and two overlays. The resource lookup framework will
query the packages in the order C, B, A.

        +------+------+-     -+------+------+
        | 0x01 |      |  ...  |      | 0x7f |
        +------+------+-     -+------+------+
            |                           |
        "android"                Target package A
                                        |
                       Pre-installed overlay B (priority 1)
                                        |
                       Pre-installed overlay C (priority 2)