vold: switch f2fs gc_urgent_high to gc_urgent_mid

We need to reduce F2FS GC intensity for smart idle maintenance service
not to interfere with other I/O activities.

Test: check smart idle maintenance log every hour
Bug: 202283480
Bug: 181079477
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Change-Id: Ideb1641a1539a23828bc8b00d9739c8128a984c0