Fix sanitizer in audioflinger Threads.cpp.

In RecordThread::start(), an unsigned integer is negated before being
set to mFramesToDrop (which is ssize_t). This causes a runtime error on
integer sanitized builds.

 runtime error: negation of 1323000 cannot be represented in type
 'unsigned int'

This makes the conversion to ssize_t explicitly occur before the
negation.

Bug: 30969751
Test: Compiles.
Change-Id: I9cf2e31728ff715c9755f257c1abf10fa5efbf82
Merged-In: I9cf2e31728ff715c9755f257c1abf10fa5efbf82