Shorten safe mode timer in VCN test mode

This change reduces the time required to test VCN safe mode in CTS, in
order to meet the 30s CTS quality bar

Bug: 193482882
Test: atest --iterations 10 android.net.vcn.cts.VcnManagerTest
Original-Change: googleplex-android-review.git.corp.google.com/15375749
Change-Id: Ibbf7132d45a7a0e5554b74fda6b3e309ee2d9c44
Merged-In: Ibbf7132d45a7a0e5554b74fda6b3e309ee2d9c44