Update preferred payment related APIs

Add API docs for ACTION_PREFERRED_PAYMENT_CHANGED and the constants(PREFERRED_PAYMENT_*).
Change return type from String to CharSequence for
getDescriptionForPreferredPaymentService.

Test: build pass
Bug: 149998286
Merged-In: Ibe0c77bb25da2673cc8f46cd1e2735c0a1e30523
Change-Id: Ibe0c77bb25da2673cc8f46cd1e2735c0a1e30523
3 files changed