Add StorageManager#openProxyFileDescriptor.

The CL:

 * Adds StorageManager#openProxyFileDescriptor API.
 * Turns IProxyFileDescriptorCallback into a class so that it can provides
   default implementations.
 * Removes mActive state from FuseAppLoop, because the state will be managed at
   the MountService side.
 * Adds StorageManagerIntegrationTest to check if FUSE is correctly mounted or
   not. Since it's implementation details, CTS does not help to test the
   behavior.

Test: StorageManagerIntegrationTest
Bug: 29970149

Change-Id: Id78dd4abcf9325820e9c77c264f54bfa77b85a92