Validate caller in SlicePermissionActivity to prevent spoofing.

Adds a check in SlicePermissionActivity's onCreate to verify the true caller. The activity will only proceed if launched by the SliceProvider's package, SystemUI, or the "android" system. This prevents Confused Deputy attacks where a malicious app could trick the user into granting slice permissions by spoofing the package name in the intent. New unit tests are added in SlicePermissionActivityTest to cover valid and invalid caller scenarios.

Bug: 486385459
Flag: EXEMPT BUGFIX
(cherry picked from commit 7f44449562bc0a09e52ec84b37d95733c823e72d)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:90e563165474eb865ba3aecd54e71a0c445966c0
Merged-In: I49822e9f7eb2d29278d1266ae2fd51fc154744db
Change-Id: I49822e9f7eb2d29278d1266ae2fd51fc154744db