DngCreator: Populate baseline exposure tag

"postRawSensitivityBoost" contains the gain value applied after
RAW. Take this value in to account and populate the baseline
exposure accordingly.

Bug: 33623101
Test: runtest -x
cts/tests/camera/src/android/hardware/camera2/cts/DngCreatorTest.java

Change-Id: Ib90a5c1791c422fd36ec44fb6ef695ba8a1dc475