Galaxys2 mc1n2: Change DNG threshold from -60 dB to -78 dB

Based on some user testing, a threshold of -60 dB is too loud and causes
DNG to activate in cases where it shouldn't.

That users' results indicate good behavior at -78 dB.

Change-Id: I9374c9c616754fb379dcf17b94618322c54b6d8a