Fix `mojo_run --android` to terminate when the shell terminates.

Fixes domokit/devtools#32.

R=etiennej@chromium.org

Review URL: https://codereview.chromium.org/1288003002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 0f1a3a42379388594fd5f40fb58b829462bf64cd

devtoolslib/android_shell.py

diff --git a/devtoolslib/android_shell.py b/devtoolslib/android_shell.py
index f5a1fa8..c3bed33 100644
--- a/devtoolslib/android_shell.py
+++ b/devtoolslib/android_shell.py
@@ -366,6 +366,7 @@
 
     logcat_watch_thread = threading.Thread(
         target=_forward_observatories_as_needed)
+    logcat_watch_thread.daemon = True
     logcat_watch_thread.start()
 
   @overrides(Shell)