Don't run exec_with_ld_config_file* tests on non-production devices

The tests that use LD_CONFIG_FILE environment variable shouldn't be run
on non-production devices because LD_CONFIG_FILE is only for debuggable
builds. We have used the build-time flag USE_LD_CONFIG_FILE, which is
set for the debuggable builds, to conditionally include or exclude the
tests. However, this can be a problem when 1) the device is not
debuggable but 2) the CTS itself is built with debuggable target. So,
instead of relying on the build-time flag USE_LD_CONFIG_FILE, the tests
now check the debuggability of the device and skip the tests when it
isn't debuggable.

Bug: 65842135
Test: 1) flash user build image to a 2017 pixel device.
2) build cts with 'userdebug' build target
3) CtsBionicTestCases pass on the device

Change-Id: Ib88e3b26d093e5479d52cd87db47dee5e108cac2