Extend watchdog timeout in case of unmounting

Unmounting public volumes through Settings app would call into unmount
method in StorageManagerService. Unmounting could take long time to
complete. Hence, calling extendWatchdogTimeout to make sure that
Watchdog won't throw given it's known that unmounting is long running
operation, that's not really blocked.

Test: Manual
Bug: 369519866
Flag: EXEMPT bugfix
Change-Id: I1a307372fc08dc6f36c479a586648defe32d3a86