Set JAVA_HOME env var to OpenJDK 8 for Android builds

[aneeshusa]

The new gradle builds require Java 8,
and the existing ant builds also work with Java 8.

This is easier than running many update-alternatives calls from Salt.
Moreover, this allows keeping Java 7 installed together with Java 8.

Needed for servo/servo#15773.
Follow-up to to #617 and #629.

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This change is 

[aneeshusa]

@MortimerGoro can you please confirm that JAVA_HOME works with the ant build, so we don't break the current Android build?

[MortimerGoro]

@aneeshusa I ran some tests in my machine:

Gradle

• Compiles ok with JAVA_HOME set to Java8 (/usr/lib/jvm/java-8-openjdk-amd64)
• Fails as expected with JAVA_HOME set to Java 7 (/usr/lib/jvm/java-7-openjdk-amd64): Unsupported major.minor version 52.0

Ant

• Compiles ok with JAVA_HOME set to Java8 (/usr/lib/jvm/java-8-openjdk-amd64)
• Compiles ok with JAVA_HOME set to Java 7 (/usr/lib/jvm/java-7-openjdk-amd64)
• Fails as expected when setting an invalid JAVA_HOME folder: Error: JAVA_HOME is not defined correctly. This confirms that ant takes JAVA_HOME into account.

My environment:

• Both java7 and java8 installed on Ubuntu 16.04.
• Both android-18 and android-25 APIs installed.
• Android tools v25.2.5. Important: Android tools >= v25.3.x breaks the old build system because they deprecated android cli command.

[MortimerGoro]

Based on these tests setting JAVA_HOME env variable seems safe.

[aneeshusa]

Thanks so much @MortimerGoro! This is ready for review, r? @larsbergstrom

[larsbergstrom]

This means that we're probably not going to support cross-compiling from OSX, right? @fabricedesre Is that OK with you?

I know that the OSX cross builds have been broken for a little while (don't test => eventually breaks), but this seems to explicitly move us down a road of Linux-only cross builds. Unless that directory should also be present on OSX, too?

Otherwise, I'm fine with this, assuming it passes a try!

[fabricedesre]

I don't personally care about cross compiling from OSX, being a linux user. go ahead!

[aneeshusa]

@larsbergstrom JAVA_HOME is not required in general, we're only using it to point to Java 8 instead of Java 7 on the builders. macOS builds should still work if only Java 8 is installed, or else I believe there are similar directories JAVA_HOME can be set to.

It does make sense to template this in the future, though, so I've added a comment to that effect.
@bors-servo r=larsbergstrom

[bors-servo]

📌 Commit ca86297 has been approved by larsbergstrom

[bors-servo]

⌛ Testing commit ca86297 with merge de820a3...

[bors-servo]

☀️ Test successful - status-travis
Approved by: larsbergstrom
Pushing de820a3 to master...