use API level 9 for arm and x86

This change makes them more compatible with Firefox releases, which
currently use API level 9 as their baseline.

Fixes #98.

Author: froydnj

master/master.cfg

@@ -861,7 +861,7 @@ class ConfigCommand(object):
 
         # Set up the path to the android NDK
         if testing_android:
-            s += " --arm-linux-androideabi-ndk=/android/ndk-arm-18"
+            s += " --arm-linux-androideabi-ndk=/android/ndk-arm-9"
             s += " --i686-linux-android-ndk=/android/ndk-x86"
             s += " --aarch64-linux-android-ndk=/android/ndk-aarch64"
             s += " --disable-docs"

slaves/android/install-ndk.sh

@@ -8,12 +8,18 @@ set -ex
 curl -O http://dl.google.com/android/ndk/android-ndk-r10e-linux-x86_64.bin
 chmod +x android-ndk-r10e-linux-x86_64.bin
 ./android-ndk-r10e-linux-x86_64.bin > /dev/null
+# We use android-9 on arm and x86 for maximum compatibility with systems
+# like Firefox.  We use 21 for aarch64 and x86-64 because that is the
+# earliest API level at which they are supported.
 bash android-ndk-r10e/build/tools/make-standalone-toolchain.sh \
-        --platform=android-18 \
+        --platform=android-9 \
         --toolchain=arm-linux-androideabi-4.8 \
-        --install-dir=/android/ndk-arm-18 \
+        --install-dir=/android/ndk-arm-9 \
         --ndk-dir=/android/android-ndk-r10e \
         --arch=arm
+# NB: this arm toolchain is used for testing the libc crate to ensure it
+# works against the most recent version of android.  It cannot be combined
+# with the previous toolchain!
 bash android-ndk-r10e/build/tools/make-standalone-toolchain.sh \
         --platform=android-21 \
         --toolchain=arm-linux-androideabi-4.8 \
@@ -27,7 +33,7 @@ bash android-ndk-r10e/build/tools/make-standalone-toolchain.sh \
         --ndk-dir=/android/android-ndk-r10e \
         --arch=arm64
 bash android-ndk-r10e/build/tools/make-standalone-toolchain.sh \
-        --platform=android-21 \
+        --platform=android-9 \
         --toolchain=x86-4.9 \
         --install-dir=/android/ndk-x86 \
         --ndk-dir=/android/android-ndk-r10e \