fs_mgr: overlay: suppress noise associated with test mounting scratch partition

Add internal fs_mgr_is_ext4 and fs_mgr_is_f2fs to get heads up on
mount failures and thus bypass trying.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Ieb1f8c19ced930b6fe2d1791ef710ce528da7e37