adb: Correctly drop caps when ambient capabilities are used

This change splits the capability-dropping step of adbd into two. This
is more robust when ambient capabilities are being used, since minijail
cannot currently handle that case.

Bug: 77146512
Test: grep Cap /proc/`pidof adbd`/status
      CapInh: 0000000000000000
      CapPrm: 0000000000000000
      CapEff: 0000000000000000
      CapBnd: 00000000000000c0
      CapAmb: 0000000000000000

Change-Id: I0476a8d80f7a2497600196932542045f3bc87537
Merged-In: I0476a8d80f7a2497600196932542045f3bc87537
(cherry picked from commit daacf4f6)