Unsupported app usage API cleanup

This CL includes a number of cleanups related to unsupported app usage
tags.

TelecomManager#EXTRA_IS_HANDOVER - public APIS exist for this usecase;
updated docs and limit to P with maxtarget sdk.
TelecomManager#getSimCallManager(int userId) - marked as max target P;
there is a public API available which can be used to accomplish the same
thing.  Nobody should be calling this.
TelecomManager#getCallCapablePhoneAccounts(boolean) - marked as max
target P; callers should use the version without the boolean parameter.

Deprecate CAPABILITY_CAN_UPGRADE_TO_VIDEO; its redundant.

New public API:
Promote TelecomManager#getSystemDialerPackage() to public API; its a
benign API and doesn't expose anything which can't be inferred other ways.

New system APIs:
TelecomManager#getCurrentTtyMode() - used by settings app.
TelecomManager#setDefaultDialer() - used by useful for settings app.
TelecomManager#TTY_MODE* constants - used by getCurrentTtyMode API.

Test: Compile / run unit tests
Bug: 119305590
Change-Id: I67d6213e6c689fb545fc9ceae4a359831f97c32f