allow adb to remount symlink mount points

Currently `adb remount` won't remount symlink mount points.
In Android Generic System Image, there is a symlink
/product -> /system/product for devices with and without a physical
/product partition to work, respectively:

  - Mount product partition under /system/product via
    'mount /product' OR

  - Keep using /product -> /system/product symlink,
    when no product partition

Currently find_proc_mount() is seeking "/product" under /proc/mounts.
But the actual mount path is "/system/product" when GSI is used
on a device with product partition.

Bug: 111539442
Test: adb remount && touch /product/abc on both GSI and non-GSI

Change-Id: I8f15a67109d0a3f4ee18596ef7eb4280c5631b11