[MOB-109] Custom Firebase messaging service support

[vbabenkoru]

Add public methods to be called from a custom FirebaseMessagingService subclass if the app has one.

[codecov]

Codecov Report

Merging #71 into master will decrease coverage by 0.35%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
- Coverage   61.65%   61.29%   -0.36%     
==========================================
  Files          26       26              
  Lines        1536     1545       +9     
  Branches      157      160       +3     
==========================================
  Hits          947      947              
- Misses        483      490       +7     
- Partials      106      108       +2

Impacted Files	Coverage Δ
...iterableapi/IterableFirebaseInstanceIDService.java	0% <0%> (ø)	⬆️
...ain/java/com/iterable/iterableapi/IterableApi.java	55.27% <0%> (-0.23%)	⬇️
...rable/iterableapi/IterableNotificationBuilder.java	77.29% <0%> (-0.43%)	⬇️
.../iterableapi/IterableFirebaseMessagingService.java	54.83% <54.54%> (-8.13%)	⬇️

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[tapashmajumder]

+1