Pull underlying network MTU using Java APIs

This change ensures that if the VCN's underlying network MTU is not
reported via the LinkProperties APIs (as in the case of Carrier WiFi),
the kernel values will be used (as possible).

Bug: 204813618
Test: Manual testing
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: If291aa6b915e4da39152a679bbe3e40216b42f44