floss: Change build rules from //bt to //bt/system

Ran a sed rule to replace most instances of //bt to //bt/system in all
BUILD.gn and *.gni files. Also made some manual changes to include
directories using packetgen and dumpsys to include bt/system/gd instead
of just bt/gd. Added a new BUILD.gn at root that just points to
system/BUILD.gn.

On the Rust side, updated the Cargo.toml paths and build.rs includes for
topshim.

Bug: 205591223
Test: Run `./build.py`
Tag: #floss
Change-Id: Ib6b9b991f140742d765a1031c8f4085aba9ce552
diff --git a/system/gd/common/BUILD.gn b/system/gd/common/BUILD.gn
index fb4dad9..7941b7a 100644
--- a/system/gd/common/BUILD.gn
+++ b/system/gd/common/BUILD.gn
@@ -22,9 +22,9 @@
     "strings.cc",
   ]
 
-  configs += [ "//bt/gd:gd_defaults" ]
+  configs += [ "//bt/system/gd:gd_defaults" ]
   deps = [
-    "//bt/gd:gd_default_deps",
-    "//bt:libbt-platform-protos-lite",
+    "//bt/system/gd:gd_default_deps",
+    "//bt/system:libbt-platform-protos-lite",
   ]
 }