Nitrous: avoid the host CPU entering runtime power mode while BT

is transferring

When BT UART is transferring, to notify cpupm driver "system busy"
in order to block the host CPU enter runtime power mode (SCID).

Bug: 185910799
Bug: 185732896
Bug: 180436653
Test: BT On/Off stress test
Change-Id: Ieaadee56a2c0f2bf62a4dc97664ca595205c6e70
Signed-off-by: jonerlin <jonerlin@google.com>