Zygote: Fix dropping capabilities in containers

Casting a int to long will sign-extend it, so if capability bit 31
was set the bitmask would always have all the upper 32 bits set,
failing to drop any of the upper capabilities.

Instead of plain casting, use Integer.toUnsignedLong to always
zero-extend the bitmask

Change-Id: I9b4c9273e8eb09f7e231b5c579e7906d05b6e475