Add signal handler to tf.

TSTP signal for quitting tradefed which waits all invocation finish.
TERM signal for killing tradefed.

Bug: 111601636
Test: manullay
Change-Id: I56bbc19d24345d8cf2fac0ee5aa969255745c59c
diff --git a/tradefed.sh b/tradefed.sh
index 23dcd5b..20c5827 100755
--- a/tradefed.sh
+++ b/tradefed.sh
@@ -23,7 +23,6 @@
 # JAVA_VERSION, RDBG_FLAG, TF_PATH, TRADEFED_OPTS
 checkPath adb
 
-
 # Note: must leave $RDBG_FLAG and $TRADEFED_OPTS unquoted so that they go away when unset
-java $RDBG_FLAG -XX:+HeapDumpOnOutOfMemoryError -XX:-OmitStackTraceInFastThrow $TRADEFED_OPTS \
+exec java $RDBG_FLAG -XX:+HeapDumpOnOutOfMemoryError -XX:-OmitStackTraceInFastThrow $TRADEFED_OPTS \
   -cp "${TF_PATH}" -DTF_JAR_DIR=${TF_JAR_DIR} com.android.tradefed.command.Console "$@"