Allow skip Android related gradle tasks

[macisamuele]

The goal of this PR is to allow development and testing of the library on systems where the Android SDK is not available.

Currently the Android dependencies are mostly needed for the sample projects, which gives an idea on how to use/integrate the library into end-user scenarios, but is not needed to test the overall library functionality.

In order to disable the inclusion/execution of the Android related tasks is sufficient to define SKIP_ANDROID environmental variable.
Anyway we want to guarantee that Android related tasks are executed on CI (almost all the public CI systems do expose CI=true environmental variable).

[cortinico]

You can remove this. If sdk.dir property is not defined in local.properties, it's up to the user to configure it. The ID or the command line will notify if the environment is misconfigured.

[macisamuele]

The idea of this PR is to conditionally include the projects depending on the presence of the android SDK.
The projects should be included if the Android SDK is available (ANDROID_SDK_ROOT env variable or sdk.dir property) or it's running on the CI

Considering this we need read the property to ensure that it is present.

[cortinico]

I can't find official docs right now, anyway the local.properties file will be recreated if missing with content of the ANDROID_SDK_ROOT. So it's sufficient to check for the env variables.

[macisamuele]

I've updated the PR reversing the logic (decide, via env variable, if android should be skipped) because there is no way to access to the content of local.properties within settings.gradle.kts.

This is relevant to me as I might define the Android SDK into the local properties file (and no env variable) and it would be annoying if Android tasks would be ignored.

About no way to access to the content, technically there is a hack that can be used (cat ... | grep sdk.dir) but it look bad enough to read a file hoping that a string is present.

So I've decided to get a different direction here.

[codecov]

Codecov Report

Merging #134 into master will decrease coverage by 0.18%.
The diff coverage is 89.58%.

@@             Coverage Diff              @@
##             master     #134      +/-   ##
============================================
- Coverage     74.56%   74.38%   -0.19%     
  Complexity      173      173              
============================================
  Files            11       11              
  Lines           684      687       +3     
  Branches         86       86              
============================================
+ Hits            510      511       +1     
- Misses          128      130       +2     
  Partials         46       46              

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/yelp/codegen/plugin/GenerateTask.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...m/yelp/codegen/plugin/GenerateTaskConfiguration.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...in/src/main/java/com/yelp/codegen/SharedCodegen.kt	61.98% <55.55%> (+0.22%)	63.00 <0.00> (ø)
.../src/main/java/com/yelp/codegen/KotlinGenerator.kt	87.61% <100.00%> (ø)	88.00 <13.00> (ø)
...ugin/plugin/src/main/java/com/yelp/codegen/Main.kt	98.38% <100.00%> (ø)	4.00 <0.00> (ø)
...rc/main/java/com/yelp/codegen/utils/StringUtils.kt	100.00% <100.00%> (ø)	6.00 <2.00> (ø)

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[macisamuele]

The change is described in #134 (comment)