mm-audio: aenc: make frame_count variable 64 bit

- After 71 minutes AAC encoding stops working
- The root cause is while calculating the timestamp
  of the frame a variable frame_count is 32 bit and
  it ovewrflows which leads incorrect timestamp
  calculation which leads to stopping of AAC encoding
- To solve this make frame_count 64 bitvariable.

Change-Id: Iddb7c6c8fab53314d58bdf8651d5ede0c6ca8b80
CRs-Fixed: 707449