Rename two "service" fields in AbstractA11yServiceConnection

(1) mService -> mClientBinder
(2) mServiceInterface -> mClient

The two "service" fields were introduced a long time ago, probably when
app-side AccessibilityServices were the only use case of the a11y
subsystem, since the word "service" refers to an AccessibilityService in
an app process.

Today the a11y subsystem supports more than just AccessibilityServices.
UiAutomation (a test API) is one example, and we will add app automation
APIs soon.

For all the above use cases, the "service" fields represent a client of
system_server, and the client can be an AccessibilityService, a
UiAutomation, etc.

For more clarity, this CL renames "service" as "client".

Bug: 369648004
Flag: EXEMPT refactor
Test: Presubmit
Test: atest CtsAccessibilityTestCases
Test: atest CtsAccessibilityServiceTestCases
Change-Id: I2118eb3f7185714bc7405c28c63c9a95a380046b