Added Android SDK version config variable

[pavrda]

If there is already more plugins in cordova project, it is useful to specify version in order to achieve each other compatibility.
I added config variable that allows to specify exact Sentry SDK version. It is possible to do that in config.xml like this:

<plugin name="sentry-cordova" spec="^0.16.2">
    <variable name="SENTRY_ANDROID_SDK_VERSION" value="1.6.8" />
</plugin>

[codecov-io]

Codecov Report

Merging #164 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   91.78%   91.78%           
=======================================
  Files           9        9           
  Lines         146      146           
  Branches       23       23           
=======================================
  Hits          134      134           
  Misses         11       11           
  Partials        1        1

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[HazAT]

Thx!