Allow admin user's app with system UID to access blob store.

BlobStore APIs are restricted to be only consumed by the admin user's
app with system uid. Right now there is only one admin user
which is user 0 and for user 0 both the appId and callingUid are same.
So even with callingUid, the existing check works well.

With the new headless mode, there can be several Admin users,
and any Admin user's app with a system uid can call BlobStoreManager APIs. But the appId and callingUid have different values on other(except user 0) amdin users' apps.
callingUid = userId + "0" + appId

Refactored the restriction to use appId instead of callingUid to match with SYSTEM_UID and added an additional check to ensure that the calling user is an admin user.

Removed unused callingUid param in internal method.

Bug: 256119753

Test: atest --test-mapping apex/blobstore
Change-Id: I6717cb8ebeb263c911e3caae924b7c3935e58778