AG: Parse BRSF as uint32_t and discard reserved bits

Per HFP v1.9 BRSF could be a 32-bit integer and all reserved bits should
be ignored; However, the original implementation responds ERROR to the
HF if it doesn't fit into int16_t.

Bug: 323470161
Bug: 323470163
Bug: 331129986
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: manual HFP/AG/SLC/BI-01-C and HFP/AG/SLC/BV-11-C on Floss
Change-Id: Ib0c5193dc19dcae37918eb61e978049ebdfcb3dd