Generalize tools/hiddenapi/checksorted_sha.sh

Stops it being specific to frameworks/base. This change works because
$2 is $(REPO_ROOT) and $(REPO_ROOT)/frameworks/base is the same as $PWD
when this script is used in the frameworks/base repo.

Also removes boot/ from the path that identifies hidden API flag files.
This keeps the directory structure needed in the repositories into
which hidden API flag files will be moved simple by removing the need
to have a boot/ directory. The hiddenapi/ directory is required as the
files need their own separate OWNERS file.

A follow up change will move this file into tools/platform-compat to
allow it to be used by other repos.

Bug: 177892522
Test: try and upload changes to hiddenapi flag files
Change-Id: Ifb3690e7c596249fda84eff82f5a53f0b1b6f991