[Android] Add .cxx directories to app template .gitignore (#161069)

Fixes (but I still need to write the draft of the blog section so don't
autoclose github 🙏 ) flutter/flutter#160372.

-Module templates [already exclude the entire `.android`
directory](https://github.com/flutter/flutter/blob/d9b7e5664604a55a3e900b0c619ee319065abb0e/packages/flutter_tools/templates/module/common/.gitignore.tmpl#L40).
-Plugin templates [already exclude
`.cxx`](https://github.com/flutter/flutter/blob/d9b7e5664604a55a3e900b0c619ee319065abb0e/packages/flutter_tools/templates/plugin_shared/android.tmpl/.gitignore).

Modified the existing gitignores with
```
nl=$'\n'
sed -i'' -e '/GeneratedPluginRegistrant.java/a\'$'\n''.cxx/'"\\${nl}"'' $(find . -path \*android/.gitignore)
```
(which was probably not the best way)

Fun fact: apparently the `.gitignore` files (including the ones we copy
to the template apps) aren't indexed for search (the `.gitignore.tmpl`
are, but not the ones that we copy exactly [such as this
one](https://github.com/flutter/flutter/blob/27ba2f279075a9a66b48ce78d94d2999b06d6e26/packages/flutter_tools/templates/app/android.tmpl/.gitignore)).

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we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
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---------

Co-authored-by: Gray Mackall <mackall@google.com>
Co-authored-by: Reid Baker <hamilton.reid.baker@gmail.com>
Co-authored-by: Greg Price <gnprice@gmail.com>

Author: 4 people

dev/a11y_assessments/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app

dev/integration_tests/android_verified_input/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app

dev/integration_tests/deferred_components_test/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # This app uses a test key and should not be used in a production app.
 # key.properties

dev/integration_tests/native_driver_test/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/to/reference-keystore

dev/integration_tests/non_nullable/android/.gitignore

@@ -5,3 +5,4 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/

dev/manual_tests/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app

dev/tracing_tests/android/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app

packages/flutter_tools/templates/app/android.tmpl/.gitignore

@@ -5,6 +5,7 @@ gradle-wrapper.jar
 /gradlew.bat
 /local.properties
 GeneratedPluginRegistrant.java
+.cxx/
 
 # Remember to never publicly share your keystore.
 # See https://flutter.dev/to/reference-keystore

packages/flutter_tools/templates/plugin/android.tmpl/.gitignore

@@ -6,3 +6,4 @@
 .DS_Store
 /build
 /captures
+.cxx/

packages/integration_test/android/.gitignore

@@ -6,3 +6,4 @@
 .DS_Store
 /build
 /captures
+.cxx/