[MOB-2433] Fix Firebase check to work without a Firebase database URL

[vbabenkoru]

🔹 Jira Ticket(s) if any

• MOB-2433

✏️ Description

The code was checking for the presence of a Firebase database URL, that is not directly necessary for push to work. This PR changes the check to get the FCM sender id, that is actually necessary for push and is a better indicator of FCM being set up correctly.

[codecov]

Codecov Report

Merging #306 (a88a4d4) into master (dabf649) will increase coverage by 1.91%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   68.51%   70.42%   +1.91%     
==========================================
  Files          60       60              
  Lines        3605     3605              
  Branches      420      421       +1     
==========================================
+ Hits         2470     2539      +69     
+ Misses        894      807      -87     
- Partials      241      259      +18     

Impacted Files	Coverage Δ
...va/com/iterable/iterableapi/IterableConstants.java	0.00% <ø> (ø)
...ain/java/com/iterable/iterableapi/IterableApi.java	64.54% <100.00%> (+0.18%)	⬆️
...able/iterableapi/IterablePushRegistrationTask.java	66.10% <100.00%> (+2.16%)	⬆️
...va/com/iterable/iterableapi/IterableApiClient.java	75.32% <0.00%> (+0.65%)	⬆️
...in/java/com/iterable/iterableapi/IterableTask.java	43.33% <0.00%> (+3.33%)	⬆️
.../java/com/iterable/iterableapi/IterableConfig.java	86.95% <0.00%> (+4.34%)	⬆️
.../iterable/iterableapi/OfflineRequestProcessor.java	81.25% <0.00%> (+12.50%)	⬆️
.../com/iterable/iterableapi/IterableTaskStorage.java	23.39% <0.00%> (+23.39%)	⬆️
...terableapi/IterableNetworkConnectivityManager.java	35.29% <0.00%> (+35.29%)	⬆️
... and 1 more

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