Subscription id handling in ReadClient is weird

[bzbarsky-apple]

Problem

In ReadClient, we:

1. Read and try to verify SubscriptionId values even if we're not a subscription. This is pretty broken.
2. Enforce that the SubscriptionId in a Subscribe Response matches the previous Report Data, and fail if it does not. The spec does not say to do this. This might need to be a spec issue instead. It's really not clear to me why the spec has a SubscriptionId in Subscribe Response at all, given that it's in the Report Data bits....
3. Have an mIsPrimingReports that seems to be used only for mSubscriptionId management and does not have clearly defined semantics. It does have a name that suggests it should be true during the entire priming read, though, which it's not....

Proposed Solution

Align with spec, make code understandable, add tests.

[yunhanw-google]

for 2, spec issue is here https://github.com/CHIP-Specifications/connectedhomeip-spec/issues/5525