Guarding against invalid permissions lists in GrantPermissionsActivity

We reverted a SafetyLabel change based on it running afoul of the list
containing empty elements. As the types are weaving in and out of Kotlin
code, the permission lists are annotated that they should contain only
non-null elements.

Linking to the bug filed for the test failure.

Bug: 270255253
Test: atest
Change-Id: I2b8d8375c4ceff37f664a41bad09955f373ff570