Add banner announcing adevice when running builds.

Test: ANDROID_QUIET_BUILD=true mm
Test: mm
Change-Id: I513591137d4866bcb824b3021269c29dec32f972

envsetup.sh

@@ -1878,6 +1878,10 @@ function _trigger_build()
       >&2 echo "Couldn't locate the top of the tree. Try setting TOP."
       return 1
     fi
+
+    if [[ -z "${ANDROID_QUIET_BUILD:-}" && -n "${ANDROID_BUILD_BANNER}" ]]; then
+      echo "$ANDROID_BUILD_BANNER"
+    fi
 )
 
 function m()