Change SubMgrTest to use public methods instead of hidden ones

The getActiveSubscriptionIdList is hidden so it cannot be used in cts
tests. Alternatively, getActiveSubscriptionInfoList is public and can be
accessed via CTS test. Change the testSetpreferredDataSubscriptionId()
test to use the public one.

Bug: 136733847
Test: CTS
Change-Id: I09950212f9f551dfb52dc55de9094ec57ad860e9
(cherry picked from commit ac15baaaebc9d1e0e7b06d63b06bc21ef727edb5)
1 file changed