Update jni to get int descriptor in native layer

ConnectivityService is going to become a mainline module which
can not access the hidden APIs. The int descriptor of a
FileDescriptor is hidden for internal use only. The Network and
NetworkUtls will be parts of CS module. The corresponding usage
should be removed. There is no way in a module to access the
descriptor, so update the jni to set a FileDescriptor to native
to get the int descriptor inside the platform.

Also, update the other references in android_net_NetUtils for
getting fd to use the NDK functions in the libnativehelper.

Bug: 170598012
Test: atest FrameworksNetTests CtsNetTestCasesLatestSdk
Test: manually connect to a VPN
Change-Id: I2143c079feac53917a6e7bf7422f3180f51437fb