Auto merge of #11278 - mitchhentges:8348-reduce-duplication, r=aneeshusa

Reduce 'android-18' duplication by putting it in $ANDROID_TARGET env variable

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 --faster` does not report any errors
- [x] These changes fix #8348

Either:
- [X] These changes do not require tests because ~~I'm lazy~~ it's a configuration change, and asserting default values is usually tedious and not very helpful

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

---

Allegedly, this broke the build [back in the day](https://github.com/servo/servo/pull/8519).
Going to see if it's a piece of cake to land now

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python/servo/command_base.py

@@ -178,6 +178,7 @@ class CommandBase(object):
         self.config["android"].setdefault("sdk", "")
         self.config["android"].setdefault("ndk", "")
         self.config["android"].setdefault("toolchain", "")
+        self.config["android"].setdefault("platform", "android-18")
         self.config["android"].setdefault("target", "arm-linux-androideabi")
 
         self.config.setdefault("gonk", {})
@@ -318,6 +319,8 @@ class CommandBase(object):
             env["ANDROID_NDK"] = self.config["android"]["ndk"]
         if self.config["android"]["toolchain"]:
             env["ANDROID_TOOLCHAIN"] = self.config["android"]["toolchain"]
+        if self.config["android"]["platform"]:
+            env["ANDROID_PLATFORM"] = self.config["android"]["platform"]
 
         if gonk:
             if self.config["gonk"]["b2g"]: