[EXO] Create a service connection for proxies

A proxy will give a privileged app access to a11y data typically available to AccessibilityServices. (Refer to the design doc linked in the comments for more information.)

A proxy will easily be able to set modify a list of service infos
depending on what a11y services/features are running on the client
device.

Introduce the system connection for a proxy:

- The service connection needs to maintain a single info that
represents this proxy-supplied list, so merge list values.

- Populate properties for each listed info that are default populated
for services installed on the device. This will prevent exceptions if
an app accesses these properties.

- Make setAccessibilityTool a SystemAPI, so a proxy can set this if it
needs to, since it cannot set this before runtime.

Test: builds, atest ProxyAccessibilityServiceConnectionTest
Bug: 241429275
Change-Id: Id99e2fa08be567738751d7a9a84f0014279924a1