android: customtabs_benchmark: fix import for py3

The run_benchmark.py was converted to py3 without running it. With
--help it fails like this:

```
  File "/path/to/run_benchmark.py", line 17, in <module>
    from . import customtabs_benchmark
ImportError: attempted relative import with no known parent package
```

Adding the path and importing as 'import customtabs_benchmark'.

Bug: 1154224
Change-Id: I5741dd0aa07685cb79b268bfc0119e40fe68523c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3226108
Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#932066}

tools/android/customtabs_benchmark/scripts/run_benchmark.py

@@ -14,8 +14,6 @@
 import sys
 import threading
 
-from . import customtabs_benchmark
-
 _SRC_PATH = os.path.abspath(os.path.join(
     os.path.dirname(__file__), '..', '..', '..', '..'))
 
@@ -25,6 +23,10 @@
 sys.path.append(os.path.join(_SRC_PATH, 'build', 'android'))
 import devil_chromium
 
+sys.path.append(
+    os.path.join(_SRC_PATH, 'tools', 'android', 'customtabs_benchmark',
+                 'scripts'))
+import customtabs_benchmark
 
 _KEYS = ['url', 'warmup', 'skip_launcher_activity', 'speculation_mode',
          'delay_to_may_launch_url', 'delay_to_launch_url', 'cold',