Use ChildServiceConnectionController.isUnbound() to prevent binding status change after unbind()

This change introduces a new feature flag, UseIsUnboundCheck, to switch
the connection status check in ChildProcessConnection.java from
isConnected() to the safer ChildServiceConnectionController.isUnbound().

The isConnected() method, which checks for a non-null service object,
can be false temporarily during connection setup before the connection
is fully established. This creates a race condition where
binding-related calls (like addStrongBinding) can proceed, but later
calls (like removeStrongBinding) might fail with an assertion error if
the connection is not actually bound, as the binding count would be
mismatched.

The mConnectionController.isUnbound() method provides a more reliable
state, as it remains false until the connection is explicitly unbound
and does not become true again. Using this check prevents assertion
failures by ensuring that binding modifications only occur on a
genuinely bound connection.

Bug: 447057423

Change-Id: Ia5f420eae8a3ad2f02243b5df1c400ee0ed9a909
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6978154
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Shin Kawamura <kawasin@google.com>
Cr-Commit-Position: refs/heads/main@{#1520988}