adb-remount-test: Only test mounts that are remounted by us

Right now we assume all RW mounts (minus /data & special FS) are
remounted by us and we apply the remount/overlayfs related checks
on them unconditionally. This would generate false positives when
a partition was RW but not remounted by us.
The test should instead check mounts that were remounted by us
(transitioned from RO to RW after adb-remount), and ignore
partitions that were already RW before running adb-remount.

Bug: 313609600
Test: adb-remount-test
Change-Id: I94e8a35775271f557790a458781657eb3b24a6f5