FileBridge: fix fd ownership mismanagement.

This patch switches everything over to ParcelFileDescriptor, but the
important part of the change is changing FileBridge.getClientSocket to
return a ParcelFileDescriptor. Previously, it returned a raw
FileDescriptor that was closed by FileBridge, and the only non-test
caller of that function was taking it and constructing a
ParcelFileDescriptor from it, which would also attempt to close the fd,
leading to an fdsan abort.

Bug: http://b/162811367
Test: atest FileBridgeTest
Test: treehugger
Change-Id: I724ea7601bb072c98895f68abc08bb0e339d1db0
(cherry picked from commit 4c392e80)