Eagerly initialize BMS in Trampoline's constructor

After multi-user BMS became a lightweight class as well, so no need to
lazily initialize it. This lays the ground for unifying Trampoline and
BMS.

Test: atest TrampolineTest
Test: adb shell bmgr backupnow <package>
Bug: 135247112
Bug: 135661048
Change-Id: Ia7f69d2ed282c6dfe6443a601f27229d43802fe6