liblp: Support sdcards in PartitionOpener.

Before ImageManager was introduced, gsid avoided using PartitionOpener
when writing to external media. PartitionOpener couldn't interact with
non-boot devices, because it prepends /dev/block/by-name. We hacked
around this in both gsid and in first-stage init, which manually detects
the problem and prepends /dev/block instead.

After the ImageManager refactoring, sdcard support broke in gsid,
because it started relying on PartitionOpener. Let's fix this by allowing
/dev/block for mmcblk* names in PartitionOpener.

Bug: 139204329
Test: fiemap_image_test gtest
Change-Id: Ic1cbdbe0a18fc09522ee38cc62b35fd8193ce250