Fix broken sdk build (/bin/sh: 1: [[: not found)

The CL aosp/2878135 introduced a new shell command to be executed as
part of `m sdk dist` builds. The new command incorrectly used bash
syntax for conditionals, i.e. `if [[ ... ]]` (double [[]]) instead of
the POSIX compliant `if [ ... ]` (single []).

This broke the sdk build on systems where /bin/sh is not a symlink to
/bin/bash:

  /bin/sh: 1: [[: not found
  error: lstat out/soong/.intermediates/exported_java_aconfig_library.jar.tmp: file does not exist

Fix the breakage by using POSIX shell syntax instead.

Bug: 316933458
Bug: 311151343
Test: m sdk dist
Change-Id: I7de53261674a8f54c4ecacf0e27aecbafcfa4048

aconfig/init.go

@@ -78,7 +78,7 @@ var (
 		blueprint.RuleParams{
 			Command: `rm -rf ${out}.tmp` +
 				`&& for cache in ${cache_files}; do ` +
-				`  if [[ -n "$$(${aconfig} dump --cache $$cache --filter=is_exported:true --format='{fully_qualified_name}')" ]]; then ` +
+				`  if [ -n "$$(${aconfig} dump --cache $$cache --filter=is_exported:true --format='{fully_qualified_name}')" ]; then ` +
 				`    ${aconfig} create-java-lib --cache $$cache --mode=exported --out ${out}.tmp; ` +
 				`  fi ` +
 				`done` +