Fix mul-overflow when converting scan result's age from ms to us

wifi_cached_scan_result's age_ms is a u32, which may cause
mul-overflow when multiplied by 1000. Fix this by explicitly
converting the type to u64 before doing multiplication.

Bug: 387187200
Test: m
Change-Id: Idaeb7bf5185b776cdeddb18c83f1b5b32e8b68a2