Support "always report signal strength" in BT pkg with new API

"Always report signal strength" was used to report system signal
strength when device is idle in Bluetooth package. The feature
is obsoleted and has been cleared up in b/177924721.

The function is supported now by the API
SignalStrengthUpdateRequest#setSystemThresholdReportingRequestedWhileIdle
newly introduced in S release.

Bug: 205585585
Test: atest HeadsetPhoneStateTest
Test: manual BT functions test to make sure no crash
Change-Id: I43340f6a3d8ad9e97019ddd416ac4823930d3931