Reduce duplication between projects/geckoview/build and projects/geckoview/build_apk

Some parts are duplicated between build and build_apk:

• the cd $builddir/[% project %]-[% c("version") %] and copy of mozconfig file
• the mach configure lines are the same, except an additional --enable-update-channel=[% c("var/variant") %] in build_apk, which is already present in mozconfig.in (so probably not necessary on the configure command)
• when doing the build_apk step the source tarball is extracted twice: one time in build_common and one time in build_apk

We can move this to the build_common file.