Make --split_apk mobile-install work with a pristine device.

--
MOS_MIGRATED_REVID=103068396
diff --git a/tools/android/incremental_install.py b/tools/android/incremental_install.py
index 088af54..04da8cd 100644
--- a/tools/android/incremental_install.py
+++ b/tools/android/incremental_install.py
@@ -178,10 +178,7 @@
     if match:
       return match.group(1)
     else:
-      raise TimestampException(
-          "Package '%s' is not installed on the device. At least one "
-          "non-incremental 'mobile-install' must precede incremental "
-          "installs." % package)
+      return None
 
   def GetAbi(self):
     """Returns the ABI the device supports."""
@@ -586,6 +583,12 @@
         "'mobile-install' must precede incremental installs")
 
   actual_timestamp = adb.GetInstallTime(app_package)
+  if actual_timestamp is None:
+    raise TimestampException(
+        "Package '%s' is not installed on the device. At least one "
+        "non-incremental 'mobile-install' must precede incremental "
+        "installs." % app_package)
+
   if actual_timestamp != expected_timestamp:
     raise TimestampException("Installed app '%s' has an unexpected timestamp. "
                              "Did you last install the app in a way other than "
diff --git a/tools/android/incremental_install_test.py b/tools/android/incremental_install_test.py
index 87500fc..e16a00b 100644
--- a/tools/android/incremental_install_test.py
+++ b/tools/android/incremental_install_test.py
@@ -86,10 +86,11 @@
       if shell_cmdln.startswith(("mkdir", "am", "monkey", "input")):
         pass
       elif shell_cmdln.startswith("dumpsys package "):
-        return self._CreatePopenMock(
-            0,
-            "firstInstallTime=%s" % self.package_timestamp,
-            "")
+        if self.package_timestamp is not None:
+          timestamp = "firstInstallTime=%s" % self.package_timestamp
+        else:
+          timestamp = ""
+        return self._CreatePopenMock(0, timestamp, "")
       elif shell_cmdln.startswith("rm"):
         file_path = shell_cmdln.split()[2]
         self.files.pop(file_path, None)