Use buffered DataInputStream vs RandomAccessFile

Switching to buffered streams saved up to 50% of
BackupManagerService.onUnlockUser time:

Before:
SystemServiceManager: Service com.android.server.backup.BackupManagerService$Lifecycle took 389 ms in onUnlockUser
SystemServiceManager: Service com.android.server.backup.BackupManagerService$Lifecycle took 380 ms in onUnlockUser
After:
SystemServiceManager: Service com.android.server.backup.BackupManagerService$Lifecycle took 178 ms in onUnlockUser
SystemServiceManager: Service com.android.server.backup.BackupManagerService$Lifecycle took 155 ms in onUnlockUser

Test: manual
Bug: 37686433
Change-Id: I41185a88e8377b434fab6a3bf1fd7086758127cd