Add support for additional Gradle plugins

Some Google Android features such as Firebase Crashlytics are enabled
with Gradle plugins. Add support for specifying additional plugins in
`build.gradle`.

doc/source/buildoptions.rst

@@ -94,6 +94,8 @@ options (this list may not be exhaustive):
 - ``--enable-google-services``: Enable the Google Services Gradle plugin.
   This option requires a ``google-services.json`` file in root of the
   project directory.
+- ``--add-gradle-plugins``: Add a plugin for gradle. The format of the option
+  is ``<plugin-id>:<classpath>``. The option can be specified multiple times.
 
 
 webview
@@ -158,6 +160,8 @@ ready.
 - ``--enable-google-services``: Enable the Google Services Gradle plugin.
   This option requires a ``google-services.json`` file in root of the
   project directory.
+- ``--add-gradle-plugins``: Add a plugin for gradle. The format of the option
+  is ``<plugin-id>:<classpath>``. The option can be specified multiple times.
 
 
 service_library
@@ -186,6 +190,8 @@ systems and frameworks.
 - ``--enable-google-services``: Enable the Google Services Gradle plugin.
   This option requires a ``google-services.json`` file in root of the
   project directory.
+- ``--add-gradle-plugin``: Add a plugin for gradle. The format of the option
+  is ``<plugin-id>:<classpath>``. The option can be specified multiple times.
 
 
 Requirements blacklist (APK size optimization)

pythonforandroid/bootstraps/common/build/build.py

@@ -551,6 +551,13 @@ def make_package(args):
     if args.enable_google_services:
         shutil.copy(args.google_services_json, 'src/google-services.json')
 
+    # Convert the gradle_plugins args to a list of dicts with id and
+    # classpath keys.
+    gradle_plugins = [
+        dict(zip(('id', 'classpath'), spec.split(':', maxsplit=1)))
+        for spec in args.gradle_plugins
+    ]
+
     # gradle build templates
     render(
         'build.tmpl.gradle',
@@ -562,6 +569,7 @@ def make_package(args):
         build_tools_version=build_tools_version,
         debug_build="debug" in args.build_mode,
         is_library=(get_bootstrap_name() == 'service_library'),
+        gradle_plugins=gradle_plugins,
         )
 
     # gradle properties
@@ -764,6 +772,12 @@ def parse_args_and_make_package(args=None):
                     default=[],
                     action='append',
                     help='Ddd a repository for gradle')
+    ap.add_argument('--add-gradle-plugin', dest='gradle_plugins',
+                    default=[],
+                    action='append',
+                    help=('Add a plugin for gradle. The format of the option '
+                          'is <plugin-id>:<classpath>. The option can be '
+                          'specified multiple times.'))
     ap.add_argument('--add-packaging-option', dest='packaging_options',
                     default=[],
                     action='append',

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

@@ -9,6 +9,9 @@ buildscript {
         {% if args.enable_google_services %}
         classpath 'com.google.gms:google-services:4.3.14'
         {% endif %}
+        {% for plugin in gradle_plugins %}
+        classpath '{{ plugin.classpath }}'
+        {% endfor %}
     }
 }
 
@@ -33,6 +36,9 @@ apply plugin: 'com.android.application'
 {% if args.enable_google_services %}
 apply plugin: 'com.google.gms.google-services'
 {% endif %}
+{% for plugin in gradle_plugins %}
+apply plugin: '{{ plugin.id }}'
+{% endfor %}
 
 android {
     compileSdkVersion {{ android_api }}