Use std::unique_ptr in FileDescriptorTable

Simplify the memory management in FileDescriptorTable by using
std::unique_ptr to hold FileDescriptorInfo objects.  Would have
prevented b/231366539.

Test: adb shell dumpsys meminfo --unreachable com.android.deskclock
Change-Id: I45fb54f7d0b80471183087d4bf20fbe2a3cc2641
2 files changed