Add method to read trunk stable flag

aconfig soong modules should be used to read the trunk stable flag.
However they are not available in the mainline branch.
So this CL adds the method to check if the trunk stable flag is enabled
or not by directly reading from DeviceConfig.
It's expected that this does not work correctly if flag value is not
stored in the DeviceConfig.
But for the time being, this method can be used to keep development.
Once the mainline branch supports the aconfig soong modules, method
added by this CL must be removed and java_aconfig_library should be
used instead.

Test: adb shell device_config put android_core_networking \
Test: com.android.net.flags.test_feature true
Test: Compares the value from java_aconfig_library and DeviceConfigUtils
Change-Id: I8e35b33a3201192e940e88f96aad3b2b6685b046