Update coex APIs based on API council review

Based on API council review:
1) CoexUnsafeChannel should be immutable and use a sentinel value for
undefined power cap.
2) Remove synchronous coex getters and notify CoexCallback
immediately after registering

Bug: 184018103
Test: atest FrameworksWifiTests FrameworksWifiApiTests
Change-Id: Id63d6640418499cba617351dd6973531ba9eca47
26 files changed