Send RFC 7231 Formatted User Agent

[f2prateek]

As per the library guidelines, we want to send a custom user agent for all requests from this library.

https://paper.dropbox.com/doc/Analytics-library-guidelines-2trBhLKQD4Soz4VatvuGR#:uid=631141099033910690508560&h2=User-Agent

The header is sent in the format analytics-android/, where version is the same version sent in context.library.version for events.

Ref: https://segment.atlassian.net/browse/LIB-381

[codecov-io]

Codecov Report

Merging #582 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #582      +/-   ##
==========================================
- Coverage   83.21%   82.93%   -0.28%     
==========================================
  Files          30       30              
  Lines        2454     2455       +1     
  Branches      282      282              
==========================================
- Hits         2042     2036       -6     
- Misses        329      335       +6     
- Partials       83       84       +1

Impacted Files	Coverage Δ
.../java/com/segment/analytics/ConnectionFactory.java	100% <100%> (ø)	⬆️
.../src/main/java/com/segment/analytics/ValueMap.java	86.39% <0%> (-2.73%)	⬇️
...ain/java/com/segment/analytics/internal/Utils.java	74.13% <0%> (-1.15%)	⬇️
...cs/src/main/java/com/segment/analytics/Traits.java	92.3% <0%> (-1.1%)	⬇️

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