Not create document under the device.

The structure of files in MtpDocumentsProvider looks like
/device/storage/files. But MtpDocumentsProvider shows files
just under the device if the device has only single storage.

It causes a problem that MtpDocumentsProvider tries to create a file
under the device. Previously it tries to create a file with storageId =
0, which means MTP device choose a storage to store the file.

Because it only happens when the device has a single storage, the file
is properly written to the device. But the database in
MtpDocumentsProvider goes into the illegal state where the file is
placed just under the device.

Bug: 32561572
Test: adb shell am instrument -w -e class com.android.mtp.MtpDocumentsProviderTest com.android.mtp.tests/com.android.mtp.TestResultInstrumentation
Change-Id: I47a373ceee8a64ba9995934317693e79d2497ee0