Update android tutorial to clarify android sdk path options

The android_sdk_repository path parameter is now optional, and for most setups it is easier to omit it and rely on the `ANDROID_HOME` environment variable. Updated this part of the tutorial to clarify this.

Closes bazelbuild#2632.

--
Reviewed-on: bazelbuild#2632
PiperOrigin-RevId: 149337570
MOS_MIGRATED_REVID=149337570

CONTRIBUTORS

@@ -57,6 +57,7 @@ Kamal Marhubi <[email protected]>
 Damien Martin-Guillerez <[email protected]>
 Carl Mastrangelo <[email protected]>
 Michajlo Matijkiw <[email protected]>
+Iain McGinniss <[email protected]>
 Julio Merino <[email protected]>
 Adam Michael <[email protected]>
 Liam Miller-Cushon <[email protected]>

site/versions/master/docs/tutorial/android-app.md

@@ -67,12 +67,25 @@ Add the following lines to your `WORKSPACE` file:
 ```python
 android_sdk_repository(
     name = "androidsdk",
-    # Replace with path to Android SDK on your system
-    path = "/Users/username/Library/Android/sdk",
-    # Replace with the Android SDK API level
-    api_level = 23,
-    # Replace with the version in sdk/build-tools/
-    build_tools_version="23.0.0"
+    # Replace with your installed Android SDK API level
+    api_level = 25
+)
+```
+
+This will use the Android SDK specified referenced by the `ANDROID_HOME`
+environment variable, and automatically detect the latest build tools
+version installed within that location.
+
+Alternatively, you can explicitly specify the location of the Android
+SDK and build tools version to use by including the `path` and
+`build_tools_version` attributes:
+
+```python
+android_sdk_repository(
+    name = "androidsdk",
+    path = "/path/to/Android/sdk",
+    api_level = 25,
+    build_tools_version = "25.0.1"
 )
 ```
 
@@ -85,8 +98,6 @@ file:
 ```python
 android_ndk_repository(
     name = "androidndk",
-    # Replace with path to Android NDK on your system
-    path = "/Users/username/Library/Android/ndk",
     # Replace with the Android NDK API level
     api_level = 21
 )
@@ -98,6 +109,10 @@ the API levels to the same value for the SDK and NDK.
 [This web page](https://developer.android.com/ndk/guides/stable_apis.html)
 contains a map from Android releases to NDK-supported API levels.
 
+Similar to `android_sdk_repository`, the path to the Android NDK is inferred from
+the `ANDROID_NDK_HOME` environment variable by default. The path can also be
+explicitly specified with a `path` attribute on `android_ndk_repository`.
+
 ## Create a BUILD file
 
 A [`BUILD` file](/docs/build-ref.html#BUILD_files) is a text file that describes