Fix AAC recording issue

Fix uninitialized variable that caused ADTS recording to incorrectly
specify LTP object in the header.

Bug: 17895547
Change-Id: I75650f4963f4f371cd191bc635e32c9ab17fa8c9
Truncate file before recording into it.

Recording into an existing file of non-zero length could leave old
data in the file after the recording ends. Best case that wastes
space, worst case it results in a file that's considered corrupt
on playback.

Change-Id: I2c77a107603b2b36790958360ff0856c2b28d677