android: Move external deps into project dir

Mirroring commit d288cb9a, stop using directories outside
of our project location to host external files. Small
changes to update_external_sources_android.sh/bat to find
glslang, spirv-tool, shaderc in "external" instead of ".."
diff --git a/build-android/update_external_sources_android.sh b/build-android/update_external_sources_android.sh
index b40a367..e728d89 100755
--- a/build-android/update_external_sources_android.sh
+++ b/build-android/update_external_sources_android.sh
@@ -20,7 +20,7 @@
 
 ANDROIDBUILDDIR=$PWD
 BUILDDIR=$ANDROIDBUILDDIR/..
-BASEDIR=$BUILDDIR/..
+BASEDIR=$BUILDDIR/external
 
 GLSLANG_REVISION=$(cat $ANDROIDBUILDDIR/glslang_revision_android)
 SPIRV_TOOLS_REVISION=$(cat $ANDROIDBUILDDIR/spirv-tools_revision_android)