Downgrade minSdkVersion to 7 to run on Nook Simple Touch

Also, disable lint abortOnError so the build completes despite
getActionBar().setDisplayHomeAsUpEnabled(true) calls (properly protected
by conditions on the API version in the code).

fixes: wallabag/android-app#25

Signed-off-by: Olivier Mehani <shtrom@ssji.net>