Add DocumentsUI unit tests to MTS

Change targetprep to SuiteApkInstaller so that the MTS suite can find the apk.

Changing the targetpreparer will not affect previous apk searching for device-tests.

Test: m mts, mts-tradefed run mts-documentsUI
Bug: 139495844
Bug: 123411735
Change-Id: Ife1f9dfcdb2265dc1e07a937b90ffe6140563171
diff --git a/tests/Android.bp b/tests/Android.bp
index 92c578d..15aac38 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -87,6 +87,7 @@
 
     test_suites: [
         "device-tests",
+        "mts",
     ],
 
     platform_apis: true,
diff --git a/tests/AndroidTest.xml b/tests/AndroidTest.xml
index 7893150..7874118 100644
--- a/tests/AndroidTest.xml
+++ b/tests/AndroidTest.xml
@@ -14,7 +14,7 @@
      limitations under the License.
 -->
 <configuration description="Runs Tests for DocumentsUI.">
-    <target_preparer class="com.android.tradefed.targetprep.TestAppInstallSetup">
+    <target_preparer class="com.android.tradefed.targetprep.suite.SuiteApkInstaller">
         <option name="test-file-name" value="DocumentsUITests.apk" />
     </target_preparer>