FAFT: Implement AdbShell which executes commands via adb

And also fix or work-around the Android related stuffs. An empty file
.android_faft_tester should be created in /mnt/stateful_partition on
the host of DUT such that FAFT test knows it is an Android tester.

BUG=chromium:516398
TEST=None

Change-Id: Idd93a55408a05c15cfc689f8627b178aae80924f
Reviewed-on: https://chromium-review.googlesource.com/290735
Commit-Ready: Wai-Hong Tam <waihong@chromium.org>
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Shelley Chen <shchen@chromium.org>
diff --git a/client/cros/faft/rpc_functions.py b/client/cros/faft/rpc_functions.py
index 69dd783..64332b1 100755
--- a/client/cros/faft/rpc_functions.py
+++ b/client/cros/faft/rpc_functions.py
@@ -123,8 +123,12 @@
                                   dev_key_path='/usr/share/vboot/devkeys',
                                   internal_disk=True)
 
-        self._tpm_handler = tpm_handler.TpmHandler()
-        self._tpm_handler.init(self._os_if)
+        # FIXME(waihong): Add back the TPM support.
+        if not self._os_if.is_android:
+            self._tpm_handler = tpm_handler.TpmHandler()
+            self._tpm_handler.init(self._os_if)
+        else:
+            self._tpm_handler = None
 
         self._cgpt_handler = cgpt_handler.CgptHandler(self._os_if)