Allow LE encryption while BR/EDR encryption is going on

LE encryption request was rejected if the stack was encrypting BR/EDR
transport. There is no such restriction in the specification. BT
controllers should be able to encrypt both the transports at the same
time.
Added a system property to all platforms to allow LE encryption when
BR/EDR encryption is going on.

Test: mmm packages/modules/Bluetooth
Bug: 330704060
Flag: EXEMPT guarded by system property
Change-Id: Ia501de06a1c3f37627b3ae1422ac27ba7f83353c