Auto merge of #15396 - zhuowei:android-extract-resources, r=larsbergstrom

android: extract resources to external storage on first launch

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If /sdcard/servo doesn't exist, extract it from the assets/ folder of the APK. This fixes issue #12129.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #12129 (github issue number if applicable).

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- [ ] There are tests for these changes OR
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I tested this on a Nexus 6P running Android 7.1: on launch, the app correctly detects that /sdcard/servo is missing, extracts the contents from assets/, and launches without force closing.

This is my first Servo pull request, so apologies in advance if I'm doing something wrong. Also, I was unable to build the entirety of Servo from source (complains about missing freetype), so I just built the Java portion separately; would that impact anything?

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