Add MtpStorageManager for monitoring filesystem events

MtpStorageManager keeps track of file information and
send notifications for new files. MtpDatabase now uses
this instead of MediaProvider for getting object information,
although some operations are still reflected into MP.

Since MtpStorageManager handles storage ids, remove
that field from StorageVolume and VolumeInfo.

Clean up a lot of the jni code for MtpDatabase.

Bug: 63143623
Test: Test every MtpOperation in a variety of situations on Linux and
Windows. Also use the shell to manipulate files. Verify that the cache
is consistent throughout, and the operations behave as expected. Verify
files created by the shell appear.
Test: adb shell am instrument -w android.mtp /android.support.test.runner.AndroidJUnitRunner
Change-Id: Id4ea810047b0c323399cd833047733e5daafb30a