Android: Improve documentation on command-line build

[Pokechu22]

The old instructions would result in this:

C:\Users\Pokechu22\source\repos\dolphin>Source\Android\gradlew assemble
To honour the JVM settings for this build a single-use Daemon process will be forked. See https://docs.gradle.org/7.4.2/userguide/gradle_daemon.html#sec:disabling_the_daemon.
Daemon will be stopped at the end of the build

FAILURE: Build failed with an exception.

* What went wrong:
Directory 'C:\Users\Pokechu22\source\repos\dolphin' does not contain a Gradle build.

A Gradle build should contain a 'settings.gradle' or 'settings.gradle.kts' file in its root directory. It may also contain a 'build.gradle' or 'build.gradle.kts' file.

To create a new Gradle build in this directory run 'gradlew init'

For more detail on the 'init' task see https://docs.gradle.org/7.4.2/userguide/build_init_plugin.html

For more detail on creating a Gradle build see https://docs.gradle.org/7.4.2/userguide/tutorial_using_tasks.html

* Try:
> Run gradlew init to create a new Gradle build in this directory.
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s

because gradlew executes in the current directory, which doesn't contain build.gradle. See also buildbot configuration.

Note that I didn't specify the exact filenames, partly because it's dependent on whether you want to use the debug build (assembleDebug) or the release build (assembleRelease), both of which are run by assemble. Also, for me, I get Source/Android/app/build/outputs/apk/debug/app-debug.apk and Source/Android/app/build/outputs/apk/release/app-release-unsigned.apk, but the buildbot gets app-release.apk (presumably because I don't have any keys set up, see #10938).