Fix metrics doesn't work if multiple tethering requested

There is a bug in TetheringMetrics that the on-going builders would be
cleared when creating new builder. This means if multiple tethering
created at the same time, only latest tethering would have metrics.

Bug: 153942334
Test: atest TetheringTests
Change-Id: I16a66790bc19072d8ffe570348bcbafa91263573
2 files changed