Fix security vulnerability when register phone accounts.

Currently if the registered self-managed phone account updated to a call
provider phone account, the enable state will be directly copied to the
updated one so that malicious app can perform call spoofing attack
without any permission requirements. Fix this by disallowing change a
self-managed phone account to a managed phone account.

Bug: 246930197
Test: CtsTelecomTestCases:SelfManagedConnectionSreviceTest
Change-Id: I8f7984cd491632b3219133044438b82ca4dec80e
Merged-In: I8f7984cd491632b3219133044438b82ca4dec80e