[roll] Roll fuchsia [bt][lib] Refactor Channel to use trait-based system

Refactor `Channel` to use a `Connection` trait for multi-backend
support. Moved socket logic to `types/channel/socket.rs`.
`Channel` now delegates data flow to the connection while holding
common properties like MTU and proxies.
Added `into_fidl_channel` to the trait to return a partially filled
FIDL channel, with assembly completed by the wrapper.

Original-Bug: b/414410187
Test: fx test bluetooth-crate-tests (All tests pass)
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1517679
Original-Revision: e15d3e378cd74cb532d91d40ccff166f566cfd73
GitOrigin-RevId: a0fdfe5208f20ffcdee14953b36ef181746ee49a
Change-Id: I447017df1f59930d09c259aa210c944258c3cb7f

stem

diff --git a/stem b/stem
index 5f344ff..d936f58 100644
--- a/stem
+++ b/stem
@@ -8,6 +8,6 @@
             manifest="manifests/platform"
             root=""
             remote="https://fuchsia.googlesource.com/fuchsia"
-            revision="2ceacf4ec5a8331d7a9cb3581b4d00e1b560fce2"/>
+            revision="e15d3e378cd74cb532d91d40ccff166f566cfd73"/>
   </imports>
 </manifest>