Health: Add fixed-read-only flag for platform_oxygen_saturation_enabled

This flag will be used to enable the oxygen saturation (SpO2) platform APIs.

Bug: 369876269
Flag: android.permission.flags.platform_oxygen_saturation_enabled
Test: New flag definition.
Change-Id: I9f7decd1171c60dddf0443a4894c04eaa1608c3f