Use custom Objects class for clients that use Java <1.7

[lognaturel]

Addresses getodk/collect#3192

Adds a hash implementation to the custom Objects class introduced in #450 to support clients that use Java <1.7.

[codecov-io]

Codecov Report

Merging #455 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #455      +/-   ##
============================================
- Coverage     49.03%   49.01%   -0.02%     
+ Complexity     2940     2939       -1     
============================================
  Files           246      246              
  Lines         13698    13699       +1     
  Branches       2650     2650              
============================================
- Hits           6717     6715       -2     
- Misses         6132     6134       +2     
- Partials        849      850       +1

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/org/javarosa/core/model/FormIndex.java	48.93% <ø> (ø)	40 <0> (ø)	⬇️
src/main/java/org/javarosa/core/util/Objects.java	33.33% <0%> (-16.67%)	3 <0> (ø)
...a/org/javarosa/core/services/PrototypeManager.java	79.16% <0%> (-8.34%)	8% <0%> (-1%)

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[nribeka]

Creating an issue for the TODO I think will be a great addition to this PR :)

[ggalmazor]

LGTM!

Monkey-patching Java in a controlled way! 😲