fs_mgr: overlayfs: resize scratch should it be too small

If scratch gets made too small (eg: fastboot flash scratch small-file)
it can not recover without a workaround.  The workaround is not
intuitive, (adb enable-verity then adb disable-verity to erase and
re-establish the proper sized scratch.  This needs to be automatic.
If we detect it is too small to support a filesystem, resize it to an
more appropriate size if it exists.

Alter unit test to check for fastboot characteristics and assumptions
associated with the scratch partition, and a test case to flash the
partition.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Icfceaa461c2bf13ca5a7dd4e34675dde8a4d251f