generate_gradle.py: Allow multiple --target and add --all

docs/android_studio.md

 # Android Studio
 
 [TOC]
 
 ## Usage
 
 ```shell
-build/android/gradle/generate_gradle.py --output-directory out-gn/Debug --target //chrome/android:chrome_public_test_apk
+build/android/gradle/generate_gradle.py --output-directory out-gn/Debug
 ```
 
-This creates a project at `out-gn/Debug/gradle`. To create elsewhere: `--project-dir foo`
+This creates a project at `out-gn/Debug/gradle`. To create elsewhere:
+
+```shell
+build/android/gradle/generate_gradle.py --output-directory out-gn/Debug --project-dir my-project
+```
+
+By default, only common targets are generated. To customize the list of targets
+to generate projects for:
+
+```shell
+build/android/gradle/generate_gradle.py --output-directory out-gn/Debug --target //some:target_apk --target //some/other:target_apk
+```
 
 For first-time Android Studio users:
 
  * Avoid running the setup wizard.
     * The wizard will force you to download unwanted SDK components to `//third_party/android_tools`.
     * To skip it, select "Cancel" when it comes up.
 
 To import the project:
 
  * Use "Import Project", and select the directory containing the generated project.
@@ skipping 72 matching lines @@
 
 ### What doesn't work (yet) ([crbug](https://bugs.chromium.org/p/chromium/issues/detail?id=620034))
 
  * JUnit Test targets
  * Better support for instrumtation tests (they are treated as non-test .apks right now)
  * Make gradle aware of resources and assets
  * Make gradle aware of native code via pointing it at the location of our .so
  * Add a mode in which gradle is responsible for generating `R.java`
  * Add support for native code editing
  * Make the "Make Project" button work correctly