Remove need for build-android.sh

[paidforby]

It should be possible to build the ouinet AAR without the need for a shell script that wraps the gradle scripts. Gradle is a fully featured scripting language intended specifically for compiling code, there should not be any need to wrap it in a shell script. We might still want portions of the "bootstrap" task from shell script, but that would only need to be run once when first setting up a development machine.

• Compile target and min sdks should move to a buildSrc or a plugin dependencies directory, there are some notes on the sdk variables in android-sdk-versions

• Build and publish all abis at the same time by default, no need to specify or loop over the script, unless you want to.

• Get versionNumber from version.txt, this is already done in one part of the gradle scripts, but not others.

• buildId, just equals branch name, probably easily write this into gradle

• The setting below could possibly be set elsewhere, e.g. in local properties? or gradle properties.

--project-dir="${ROOT}"/android \
--gradle-user-home "${DIR}"/_gradle-home \
--project-cache-dir "${GRADLE_BUILDDIR}"/_gradle-cache \