Enable FuseBridgeLoop to accept new mount point after starting

The CL turns StartFuseBridgeLoop function into FuseBridgeLoop class, and
adds a method adding new appfuse mount to the loop.

After doing this, one FuseBridgeLoop can process FUSE commands from
multiple AppFuse mounts.

Bug: 34903085
Test: libappfuse_test
Change-Id: I54f11f54bc26c551281b9c32e9bb91f8f043774c
4 files changed