Remove kImageIsImmutable_Flag.

Remove kImageIsImmutable_Flag.

This flag was only used when setting or checking SkBitmap's
immutability when it did not have an SkPixelRef. Now that an
SkBitmap *must* have one in order to draw (e.g. you can no
longer have an SkBitmap that owns its pixels directly), its
immutabity without an SkPixelRef makes no sense.

Also, now that the flags are not contiguous starting from
0x01, use a more appropriate check to ensure only meaningful
flags are used.

R=reed@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/387083002