[app_installer] Refactor and add app_installer_unittests target.

testing/buildbot/chromium.win.json

 {
   "XP Tests (1)": {
     "gtest_tests": [
       "accessibility_unittests",
       {
         "test": "base_unittests",
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       },
@@ skipping 191 matching lines @@
         "script": "telemetry_unittests.py"
       },
       {
         "name": "telemetry_perf_unittests",
         "script": "telemetry_perf_unittests.py"
       }
     ]
   },
   "Win7 Tests (1)": {
     "gtest_tests": [
+      "app_installer_unittests",
       "accessibility_unittests",
       {
         "test": "ash_unittests",
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       },
       "app_shell_browsertests",
       "app_shell_unittests",
       "aura_unittests",
@@ skipping 318 matching lines @@
       "views_unittests",
       "wm_unittests",
       "aura_unittests",
       "ash_unittests",
       "compositor_unittests",
       "content_browsertests",
       "events_unittests"
     ]
   }
 }