Adding FUSE loop thread in app side.

The CL adds FuseAppLoop internal class to the framework. It parses FUSE commands
from the proxy in the system service and invokes a callback provided by
application.

Bug: 29970149
Test: None, will be tested with CTS for StorageManager#openProxyFileDescriptor.
Change-Id: I10b2add4c2743fb91eae3cb194f55843a74fb668