Update Android documentation and add -upload-only flag

Default to 64-bit instead of 32-bit in the documentation. Also the NDK's
minimum API level is 21 nowadays. Finally add some notes on how to run
the resulting binaries. I was originally going to give adb instructions
(what I usually use) but that doesn't work for tests with test data, so
add a -upload-only flag to the script to help package everything.

Change-Id: I6fa7113679506e6f53b953df9146e7cf26b82e24
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/98449
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

BUILDING.md

@@ -80,17 +80,22 @@ version r16b of the NDK.
 Unpack the Android NDK somewhere and export `ANDROID_NDK` to point to the
 directory. Then run CMake like this:
 
-    cmake -DANDROID_ABI=armeabi-v7a \
-          -DANDROID_PLATFORM=android-19 \
+    cmake -DANDROID_ABI=arm64-v8a \
+          -DANDROID_PLATFORM=android-21 \
           -DCMAKE_TOOLCHAIN_FILE=${ANDROID_NDK}/build/cmake/android.toolchain.cmake \
           -GNinja -B build
 
 Once you've run that, Ninja should produce Android-compatible binaries.  You
-can replace `armeabi-v7a` in the above with `arm64-v8a` and use API level 21 or
-higher to build aarch64 binaries.
+can replace `arm64-v8a` with `armeabi-v7a` in the above to build aarch64
+binaries.
 
 For other options, see the documentation in the toolchain file.
 
+To run tests, you can use the `util/run_android_tests.go` script. Individual
+binaries can also be pushed to the device with `adb`, but note that some tests
+require test data. The `-upload-only` flag to `run_android_tests.go` may be
+useful.
+
 To debug the resulting binaries on an Android device with `gdb`, run the
 commands below. Replace `ARCH` with the architecture of the target device, e.g.
 `arm` or `arm64`.

util/run_android_tests.go

@@ -46,6 +46,7 @@ var (
 	allTestsArgs = flag.String("all-tests-args", "", "Specifies space-separated arguments to pass to all_tests.go")
 	runnerArgs   = flag.String("runner-args", "", "Specifies space-separated arguments to pass to ssl/test/runner")
 	jsonOutput   = flag.String("json-output", "", "The file to output JSON results to.")
+	uploadOnly   = flag.Bool("upload-only", false, "Skips running the tests and only uploads the data to /data/local/tmp/boringssl/tmp on the device.")
 )
 
 func enableUnitTests() bool {
@@ -410,6 +411,11 @@ func main() {
 		os.Exit(1)
 	}
 
+	if *uploadOnly {
+		fmt.Printf("Tests and data are now uploaded to /data/local/tmp/boringssl-tmp on the device.\n")
+		return
+	}
+
 	var unitTestExit int
 	if enableUnitTests() {
 		fmt.Printf("Running unit tests...\n")