gh-116622: Add Android testbed (GH-117878)

Add code and config for a minimal Android app, and instructions to build and run it.
Improve Android build instructions in general.
Add a tool subcommand to download the Gradle wrapper (with its binary blob). Android
studio must be downloaded manually (due to the license).

Android/testbed/app/src/main/python/main.py

@@ -0,0 +1,17 @@
+import runpy
+import signal
+import sys
+
+# Some tests use SIGUSR1, but that's blocked by default in an Android app in
+# order to make it available to `sigwait` in the "Signal Catcher" thread. That
+# thread's functionality is only relevant to the JVM ("forcing GC (no HPROF) and
+# profile save"), so disabling it should not weaken the tests.
+signal.pthread_sigmask(signal.SIG_UNBLOCK, [signal.SIGUSR1])
+
+# To run specific tests, or pass any other arguments to the test suite, edit
+# this command line.
+sys.argv[1:] = [
+    "--use", "all,-cpu",
+    "--verbose3",
+]
+runpy.run_module("test")