Make writing Bitmap to ashmem faster

Write to memfd instead.
Avoid mmap+memcpy+munmap.

See b/407422742 for rationale, research, and benchmark results.

Bug: 407422742
Flag: com.android.graphics.hwui.flags.bitmap_use_memfd
Test: BitmapPerfTest
Change-Id: I64c6850721004f3f6ec71964169230f8c9828e16