Protect invalid entitlement app configuration

There is a protection in Settings that Settings would gray out if
tethering requires entitlement check but the entitlement app is
invalid. Tethering resource is moved from framework to tethering
module, so Settings can not fetch entitlement app name anymore.

In this change, tethering module would check whether entitltement app
package name is exsited if entitlement check is needed. Tethering would
be not supported (Settings tethering option would be hidded) if
entitlement app is not installed.

After moving the protection into tethering module,
TetherUtil#isProvisioningNeeded is no longer needed. Because The
only use case is Settings wants to gray out tethering setting
when entitltement check is needed but entitlement app is invalid.

Bug: 146918263
Test: atest TetheringCoverageTests
Change-Id: I9a5ff5dbc1db3f3be7fcd7146862a16b373507e6