Update target API level to 37

[mhsmith]

Checklist in product/gradle-plugin/README.md.

Known issues:

• https://developer.android.com/about/versions/17/behavior-changes-17#safer-dcl:

  All native files loaded using System.load() must be marked as read-only. Otherwise, the system throws UnsatisfiedLinkError.

• Pick up any further edge-to-edge changes from the Briefcase template since 08-04, aiming to make both implementations as similar as possible.

[mhsmith]

All native files loaded using System.load() must be marked as read-only. Otherwise, the system throws UnsatisfiedLinkError.

Based on my tests, it looks like this is enforced at the Java level, so it doesn't affect us yet. For example, I can run the brotli tests no problem, but if I pass the brotli .so path to System.load in the Python console, I get this:

java.lang.UnsatisfiedLinkError: Attempt to load writable file: /data/data/com.chaquo.python.pkgtest3/files/chaquopy/AssetFinder/requirements/_brotli.so
	at java.lang.Runtime.load0(Runtime.java:956)
	at java.lang.System.load(System.java:1728)

It's likely they'll tighten this in the future, so we should fix it now anyway.