Delete attribute useTargetActivityForQuickAccess.

In practice, the value of this xml attribute is always going to a
function of  whether or not getTargetActivityPendingIntent() is null--
when a QuickAccessWalletService sends us a PendingIntent, we will use
that instead of the SystemUI card switcher activity.

If the PendingIntent is null, we then fall back to our old behavior:
* If the wallet is not currently showing any cards, launch the activity
  specified by getWalletIntent() (this is hardcoded in XML metadata)
    * If the wallet is currently showing cards, launch the SysUI
      switcher activity

Test: atest CtsQuickAccessWalletTestCases --retry-any-failure
Test: atest QuickAccessWalletControllerTest
Bug: 218860062
Change-Id: I4cfaa5b6035499c47a0ed8b1a4a5f3e1f0f50860