Add WRITE_APN_SETTINGS to shell identity

- This is required for CTS that tests APN insertion and deletion without
  carrier privileges.

Test: atest ApnReadPhoneStateTest.java (from ag/21152460)
Test: atest EmergencyCallsTest.java
Bug: 263485311

Change-Id: I8acd3d793d2288e0fa315c4d02071ecba4e8d1f6
Merged-In: I8acd3d793d2288e0fa315c4d02071ecba4e8d1f6