Add xml configuration of RROs

This change adds the ability to configure the priority, default enable
state, and mutability (previously know as staticness) of an overlay.
Rather than overlays configuring themselves, the system can configure
overlays relative to each other.

An example configuration file looks like:
<config>
    <merge path="auto-generated.xml" />
    <overlay package="com.example.one" mutable="false"
             enabled="true"/>
    <overlay package="com.example.two" mutable="false"
             enabled="true"/>
    <overlay package="com.example.three" enabled="true"/>
</config>

The <overlay> tag configures the overlay while the <merge> tag allows
additional configuration files to be included at a position within
the configuration file.

If the configuration file is not present for a partition, the legacy
android:isStatic and android:priority will continue to configure the
overlays in the partition. Once at least one configuration file has
been defined in any partition, strict partition precedence will be
enforced and overlays on separate partitions will no longer be able
to use android:priority to reorder themselves conversely from the
overlay partition precedence.

The order of the system partitions from least to greatest precedence
is system, vendor, odm, oem, product, system_ext.

Bug: 135048762
Test: atest OverlayConfigTest
Change-Id: If57e8caa9b881f9d424ef48bba80b18cc8b7b943