mm-audio: aenc-aac: fix integer overflow for encoded buffer timestamp calculation

Encoded buffer timestamp is 64 bit wide and is derived by multiplying
two 32 bit values, while calculating timestamp, these values are
multiplied with result stored in intermediate storage of 32 bit used
by compiler internally, this was associated with overflow if the result
is larger than max supported by 32 bit storage.
Changing m_frame_count to 64 bit will fix possible overflow for frame
count as well as force compiler to use intermediate variable of
required size.

Change-Id: I5ab709094d1c6b984d27f98327d3d8afa179fc92
CRs-Fixed: 767185