adb shouldn't trace unless told to

Prevents clean unmount of /data in crypto bounce

Bug: 21516860
Change-Id: I1f761dfdf216dcb35b4609cd46cc4d0644cc4a81

adb/adb.cpp

@@ -190,6 +190,13 @@ static void setup_trace_mask() {
 }
 
 void adb_trace_init(char** argv) {
+    // Don't open log file if no tracing, since this will block
+    // the crypto unmount of /data
+    const std::string trace_setting = get_trace_setting();
+    if (trace_setting.empty()) {
+        return;
+    }
+
 #if !ADB_HOST
     if (isatty(STDOUT_FILENO) == 0) {
         start_device_log();