Auto merge of #18680 - MortimerGoro:android_flavors, r=larsbergstrom

Implement Gradle flavors for Android VR compilations

<!-- Please describe your changes on the following line: -->

This PR adds support to easily generate Android VR builds. Rust/Java VR dependencies are not added by default.

Default build (No VR support)
```
./mach build --release --android
./mach package --release --android
./mach install --release --android
```

GoogleVR builds (e.g. Daydream)
```
./mach build --release --android --features googlevr
./mach package --release --android --flavor googlevr
./mach install --release --android
```

OculusVR builds (e.g. Gear VR)
```
./mach build --release --android --features oculusvr
./mach package --release --android --flavor oculusvr
./mach install --release --android
```

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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python/servo/build_commands.py

@@ -309,6 +309,11 @@ class MachCommands(CommandBase):
             env['CPPFLAGS'] = ' '.join(["--sysroot", env['ANDROID_SYSROOT']])
             env["CMAKE_ANDROID_ARCH_ABI"] = self.config["android"]["lib"]
             env["CMAKE_TOOLCHAIN_FILE"] = path.join(self.android_support_dir(), "toolchain.cmake")
+            # Set output dir for gradle aar files
+            aar_out_dir = self.android_aar_dir()
+            if not os.path.exists(aar_out_dir):
+                os.makedirs(aar_out_dir)
+            env["AAR_OUT_DIR"] = aar_out_dir
 
         cargo_binary = "cargo" + BIN_SUFFIX