DngCreator: Do not cache timezone in static initializer

The static initializer for a pre-loaded framework class is run
no later than at zygote startup, which happens at device boot.

Which means that if the timezone later changes, DngCreator will use
an incorrect cached timezone until the next reboot.  This is especially
evident in freshly wiped devices, where initial setup will typically
change the timezone.

Instead, read the timezone each time DngCreator is created, which is
also when we query the current time for constructing the timestamps.

Test: android.hardware.camera2.cts.DngCreatorTest#testSingleImageThumbnail
Bug: 31743060
Change-Id: I83a4eac762650e5f904f3b8fa779c094cef30562