Fix TetheringServiceTest test WRITE_SETTINGS permission failure

AdoptShellPermissionIdentity can not pass permission check by
Settings#checkAndNoteWriteSettingsOperation. It would compare the caller
uid and its package name. See error below:
1. java.lang.SecurityException:
Specified package com.android.shell under uid 10239 but it is really 2000
2. java.lang.SecurityException:
uid 10245 does not have android.permission.UPDATE_APP_OPS_STATS.

Override the method and test if caller hold WRITE_SETTINGS directly.

Bug: 154869719
Test: TetheringTests, TetheringCoverageTests, NetworkStackNextTests,
NetworkStackCoverageTests

Change-Id: I2a60c4d66ef30028f9663159f85464ea815248e2