decouple compileSdk version from build tools version (facebook#21205)

Summary:
Android CI was failing due to that it was trying to extract major version from build tools and use it to compile ReactAndroid. Now, it'll extract compileSdkVersion from ReactAndroid/build.gradle and use it.

Issue was that facebook@68c7999 dowgraded compileSdk version to 26 while retaining build tools version as 27.0.3, so CI was trying to use SDK 27.
Pull Request resolved: facebook#21205

Differential Revision: D9943909

Pulled By: hramos

fbshipit-source-id: ec9bc0c40956a16f8088532340722fd43cadde37

scripts/validate-android-test-env.sh

@@ -56,7 +56,7 @@ fi
 BUILD_TOOLS_VERSION=`grep buildToolsVersion $(dirname $0)/../ReactAndroid/build.gradle | sed 's/^[^"]*\"//' | sed 's/"//'`
 
 # MAJOR is something like "23"
-MAJOR=`echo $BUILD_TOOLS_VERSION | sed 's/\..*//'`
+MAJOR=`grep compileSdkVersion $(dirname $0)/../ReactAndroid/build.gradle | sed 's/[^[:digit:]]//g'`
 
 # Check that we have the right major version of the Android SDK.
 PLATFORM_DIR="$ANDROID_HOME/platforms/android-$MAJOR"