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