Fix flaky test BinderTest.TetherGetStats

It is caused by the same interface name.
We used arc4random_uniform(10000) to generate interface
name before. Change it to make sure we won't use the same name.

Error log:
05-22 10:08:56 V/GTestResultParser: [ RUN      ] BinderTest.TetherGetStats
05-22 10:08:56 V/GTestResultParser: iptables: Chain already exists.
05-22 10:08:56 V/GTestResultParser: ip6tables: Chain already exists.
05-22 10:08:56 V/GTestResultParser: system/netd/tests/binder_test.cpp:1064: Failure
05-22 10:08:56 V/GTestResultParser: Expected equality of these values:
05-22 10:08:56 V/GTestResultParser: expected1
05-22 10:08:56 V/GTestResultParser: Which is: { 975, 242, 579, 121 }
05-22 10:08:56 V/GTestResultParser: getStatsVectorByIf(statsVec, extIface1)
05-22 10:08:56 V/GTestResultParser: Which is: { 8975, 761, 5579, 509 }
05-22 10:08:56 V/GTestResultParser: system/netd/tests/binder_test.cpp:1066: Failure
05-22 10:08:56 V/GTestResultParser: Expected equality of these values:
05-22 10:08:56 V/GTestResultParser: expected2
05-22 10:08:56 V/GTestResultParser: Which is: { 8000, 519, 5000, 388 }
05-22 10:08:56 V/GTestResultParser: getStatsVectorByIf(statsVec, extIface2)
05-22 10:08:56 V/GTestResultParser: Which is: { 8975, 761, 5579, 509 }

Bug: 133818065
Test: cd system/netd && atest
Change-Id: I228f769d070e39eb6bb0b5e46a084b3db29a3b84