Get --add-source working for dirs in Gradle builds (kivy#2156)

* Switch to using Gradle srcDir props for extra source dirs.

* Add warning about only supporting source dir includes with gradle.

* Fixed typos in gradle source directory warning

Co-authored-by: Alexander Taylor <alexanderjohntaylor@gmail.com>

pythonforandroid/bootstraps/common/build/build.py

@@ -380,6 +380,9 @@ def make_package(args):
         for spec in args.extra_source_dirs:
             if ':' in spec:
                 specdir, specincludes = spec.split(':')
+                print('WARNING: Currently gradle builds only support including source '
+                      'directories, so when building using gradle all files in '
+                      '{} will be included.'.format(specdir))
             else:
                 specdir = spec
                 specincludes = '**'

pythonforandroid/bootstraps/common/build/templates/build.tmpl.gradle

@@ -85,6 +85,13 @@ android {
     sourceSets {
         main {
             jniLibs.srcDir 'libs'
+            java {
+
+                {%- for adir, pattern in args.extra_source_dirs -%}
+                    srcDir '{{adir}}'
+                {%- endfor -%}
+
+            }
         }
     }