Override module's C flags when coverage is enabled

Bug: http://b/116873221

Disable -Wframe-larger-than because building without optimization can
make frame larger.  Building dng_sdk with coverage results in a bigger
stack frame than the limit in C flags.

Get optimization level back to -O0 if module has a non-default
optimization level.  Not doing so causes link failures with
libneuralnetworks.so.

Test: Build with coverage.
Change-Id: I36b2979bb6a73023458c8de3241e975845bd36eb