Make getPhoneNumber methods safer and faster

Make the getPhoneNumber() methods a little bit safer
and easier to work with by having both APIs call a
single internal utility method. This eliminates redundant
permissions checks that could all result from a single
method call to getPhoneNumber. In very rare cases, these
permissions checks could have resulted in inconsistent
results due to context switching.

Also fix an issue where DEFAULT_SUBSCRIPTION_ID had some
inconsistent behavior. It would work "correctly" for
PHONE_NUMBER_SOURCE_UICC but wouldn't for the other two
sources.

Bug: 317673478
Test: atest FrameworksTelephonyTests
Change-Id: I53fe0e7afefe1f2bc7d280fa74768b77962bc078