AudioRecord::getInputFramesLost() cleanup

Fixed bug that if the binder call failed (for example if the
IAudioFlinger binder is dead), then getInputFramesLost was returning
garbage.  Now it correctly returns zero, which is the error value for
this method.

The type declarations for getInputFramesLost were inconsistent:
a mixture of unsigned int, size_t, and uint32_t.  Now it returns uint32_t
everywhere, which is what the underlying HAL API returns.

Added a FIXME about the side effect behavior.  This will need review
for multi-client.

Change-Id: Ifa2e117a87dbd0c1f2c892a31d1c3dd919bf1a0a