Ensure adb key store is instantiated before revoking grants

If adb is not enabled on a device then the adb key store is not
instantiated when the AdbDebuggingManager's handler is started as
in most cases it will not be needed. However if the user attempts
to revoke any previous adb grants when a device is booted with adb
disabled it will result in a NPE. This commit ensures the key store
is instantiated before attempting to revoke authorizations.

Test: atest AdbDebuggingManagerTest
Test: Manually invoked 'Revoke USB debugging authorizations' after
      a boot with adb disabled.
Fixes: 145790817

Change-Id: Id00e531c18412be009a211eec80a72867659608f