fifo: fix bugs in lost frames calculation

Problems fixed:
 - did not set the lost frames parameter to zero when no frames were lost
 - under-estimated lost frames by mFrameCount

Also improved the documentation about lost frames.

Test: currently using interactive test only, need an automated version
Change-Id: I8b056c9cf4ead0d197b4c3aeff8482a08710c4cb
2 files changed